ISYNA1

gene
On this page

Also known as Ino1INOSIPS

Summary

ISYNA1 (inositol-3-phosphate synthase 1, HGNC:29821) is a protein-coding gene on chromosome 19p13.11, encoding Inositol-3-phosphate synthase 1 (Q9NPH2). Key enzyme in myo-inositol biosynthesis pathway that catalyzes the conversion of glucose 6-phosphate to 1-myo-inositol 1-phosphate in a NAD-dependent manner.

This gene encodes an inositol-3-phosphate synthase enzyme. The encoded protein plays a critical role in the myo-inositol biosynthesis pathway by catalyzing the rate-limiting conversion of glucose 6-phosphate to myoinositol 1-phosphate. Alternatively spliced transcript variants encoding multiple isoforms have been observed for this gene, and a pseudogene of this gene is located on the short arm of chromosome 4.

Source: NCBI Gene 51477 — RefSeq curated summary.

At a glance

  • GWAS associations: 3
  • Clinical variants (ClinVar): 93 total
  • MANE Select transcript: NM_016368

Identifiers

Gene identifiers

FieldValue
HGNC IDHGNC:29821
Approved symbolISYNA1
Nameinositol-3-phosphate synthase 1
Location19p13.11
Locus typegene with protein product
StatusApproved
AliasesIno1, INOS, IPS
Ensembl geneENSG00000105655
Ensembl biotypeprotein_coding
OMIM611670
Entrez51477

Gene structure

Transcript identifiers

Ensembl transcripts: 32 — 25 protein_coding, 4 retained_intron, 3 nonsense_mediated_decay

ENST00000338128, ENST00000457269, ENST00000577820, ENST00000577916, ENST00000578352, ENST00000578963, ENST00000581672, ENST00000581800, ENST00000582287, ENST00000582770, ENST00000582811, ENST00000583309, ENST00000583534, ENST00000583816, ENST00000885772, ENST00000885773, ENST00000885774, ENST00000885775, ENST00000885776, ENST00000885777, ENST00000915490, ENST00000915491, ENST00000915492, ENST00000915493, ENST00000915494, ENST00000915495, ENST00000915496, ENST00000915497, ENST00000915498, ENST00000915499, ENST00000970148, ENST00000970149

RefSeq mRNA: 3 — MANE Select: NM_016368 NM_001170938, NM_001253389, NM_016368

CCDS: CCDS12379, CCDS54234, CCDS62603

Canonical transcript exons

ENST00000338128 — 11 exons

ExonStartEnd
ENSE000022852391843438818435117
ENSE000023212921843809318438133
ENSE000035102491843697318437105
ENSE000035845721843786018437988
ENSE000036244461843759918437760
ENSE000036955061843603218436247
ENSE000036965041843668418436877
ENSE000036965711843526618435483
ENSE000036969411843556318435676
ENSE000036984291843575718435921
ENSE000037005581843633018436479

Expression profiles

Bgee: expression breadth ubiquitous, 263 present calls, max score 99.61.

FANTOM5 (CAGE): breadth ubiquitous, TPM avg 39.4757 / max 454.2426, expressed in 1525 samples.

FANTOM5 promoters (4 alternative TSS)

Promoter IDTPM avgSamples expressed
18000335.15941502
1800043.9687992
1800020.2678111
1800050.079757

Top tissues by expression

283 total, by Bgee expression score (0-100, higher = more expressed):

TissueAnatomy IDExpression scoreQuality
right testisUBERON:000453499.61gold quality
right uterine tubeUBERON:000130299.58gold quality
left testisUBERON:000453399.56gold quality
testisUBERON:000047398.28gold quality
right ovaryUBERON:000211897.81gold quality
left ovaryUBERON:000211997.78gold quality
left uterine tubeUBERON:000130397.35gold quality
ventricular zoneUBERON:000305397.14gold quality
right coronary arteryUBERON:000162597.01gold quality
apex of heartUBERON:000209896.91gold quality
metanephros cortexUBERON:001053396.91gold quality
left coronary arteryUBERON:000162696.84gold quality
adenohypophysisUBERON:000219696.77gold quality
cortical plateUBERON:000534396.57gold quality
coronary arteryUBERON:000162196.44gold quality
embryoUBERON:000092296.35gold quality
endocervixUBERON:000045896.00gold quality
ganglionic eminenceUBERON:000402395.97gold quality
pituitary glandUBERON:000000795.59gold quality
body of uterusUBERON:000985395.47gold quality
ectocervixUBERON:001224995.12gold quality
ovaryUBERON:000099295.01gold quality
adult organismUBERON:000702395.00gold quality
olfactory segment of nasal mucosaUBERON:000538694.98gold quality
descending thoracic aortaUBERON:000234594.69gold quality
ascending aortaUBERON:000149694.58gold quality
thoracic aortaUBERON:000151594.54gold quality
body of pancreasUBERON:000115094.51gold quality
aortaUBERON:000094794.29gold quality
tibial arteryUBERON:000761094.27gold quality

Single-cell (SCXA)

Detected in 7 experiment(s), a significant marker in 7.

ExperimentMarker?Max mean expression
E-MTAB-6701yes1281.38
E-HCAD-10yes1170.70
E-GEOD-134144yes32.31
E-MTAB-9067yes22.29
E-HCAD-11yes20.64
E-ANND-3yes11.96
E-CURD-112yes10.05

Regulation

Is transcription factor: no

Upstream regulators (CollecTRI, top): CTNNB1, E2F1, IER2, MYOG, SMARCA1, SNAI1, SSRP1, TBP

miRNA regulators (miRDB)

16 targeting ISYNA1, top 30 by miRDB confidence (max_score; target_count = how many genes the miRNA targets in total — lower means more specific):

miRNAMax scoreAvg scoremiRNA target_count
HSA-MIR-627-3P99.9071.423316
HSA-MIR-444799.8567.812900
HSA-MIR-6861-3P99.6068.46444
HSA-MIR-447299.5666.081478
HSA-MIR-642A-5P99.5165.101152
HSA-MIR-6727-3P99.4965.921333
HSA-MIR-4722-3P99.3565.221099
HSA-MIR-319999.1765.19696
HSA-MIR-805299.1765.01719
HSA-MIR-429299.1665.571767
HSA-MIR-6791-5P99.1665.921844
HSA-MIR-328-5P99.0864.651000
HSA-MIR-6885-5P98.7164.33902
HSA-MIR-4646-3P98.6566.98693
HSA-MIR-4433B-5P95.9166.56727
HSA-MIR-6889-5P90.2664.13291

Literature-anchored findings (GeneRIF, showing 8)

  • Upregulation occurs through the interaction of several low-affinity E2F binding motifs present in the minimal promoter. (PMID:15464731)
  • MIP synthase expressed in developing fetal liver and shows optima pH between 7.0 and 7.5. (PMID:22707096)
  • analysis of how myo-inositol-3-phosphate synthase is regulated by phosphorylation (PMID:23902760)
  • Abnormal expression of ISYNA1 may play an important role in the progression of pancreatic ductal adenocarcinoma (PMID:30861650)
  • The ISYNA1 was an important regulatory factors and related networks were involved in multiple functional processes. (PMID:31495492)
  • Musashi2 promotes the progression of pancreatic cancer through a novel ISYNA1-p21/ZEB-1 pathway. (PMID:32779876)
  • Cigarette smoke induces the ROS accumulation and iNOS activation through deactivation of Nrf-2/SIRT3 axis to mediate the human bronchial epithelium ferroptosis. (PMID:36871899)
  • The myo-inositol biosynthesis rate-limiting enzyme ISYNA1 suppresses the stemness of ovarian cancer via Notch1 pathway. (PMID:37105506)

Cross-species orthologs

4 orthologs

OrganismSymbolGene ID
mus_musculusIsyna1ENSMUSG00000019139
rattus_norvegicusIsyna1ENSRNOG00000019741
drosophila_melanogasterInosFBGN0025885
caenorhabditis_elegansWBGENE00012148

Protein

Protein identifiers

Inositol-3-phosphate synthase 1Q9NPH2 (reviewed: Q9NPH2)

Alternative names: Myo-inositol 1-phosphate synthase, Myo-inositol 1-phosphate synthase A1

All UniProt accessions (8): Q9NPH2, A0A140VK73, J3KRH4, J3QKW9, J3QLD7, J3QRH1, J3QRY0, J3QS51

UniProt curated annotations — full annotation on UniProt →

Function. Key enzyme in myo-inositol biosynthesis pathway that catalyzes the conversion of glucose 6-phosphate to 1-myo-inositol 1-phosphate in a NAD-dependent manner. Rate-limiting enzyme in the synthesis of all inositol-containing compounds.

Subcellular location. Cytoplasm.

Tissue specificity. Highly expressed in testis, ovary, heart, placenta and pancreas. Weakly expressed in blood leukocyte, thymus, skeletal muscle and colon.

Post-translational modifications. Phosphorylation at Ser-279 and Ser-357 may be associated with a decrease in activity.

Activity regulation. Inhibited by mood-stabilizing drugs such as valproate (VPA) and lithium.

Induction. By glucose and lovastain. Up-regulation is prevented by mevalonic acid, farnesol, and geranylgeraniol. Up-regulated by E2F1.

Pathway. Polyol metabolism; myo-inositol biosynthesis; myo-inositol from D-glucose 6-phosphate: step 1/2.

Similarity. Belongs to the myo-inositol 1-phosphate synthase family.

Isoforms (3)

UniProt IDNamesCanonical?
Q9NPH2-11yes
Q9NPH2-22
Q9NPH2-33

RefSeq proteins (3): NP_001164409, NP_001240318, NP_057452* (*=MANE)

Domains & families (InterPro)

IDNameType
IPR002587Myo-inos-1-P_SynthaseFamily
IPR013021Myo-inos-1-P_Synthase_GAPDHDomain
IPR036291NAD(P)-bd_dom_sfHomologous_superfamily

Pfam: PF01658, PF07994

Enzyme classification (BRENDA):

  • EC 5.5.1.4 — inositol-3-phosphate synthase (BRENDA: 80 organisms, 52 substrates, 121 inhibitors, 63 Km, 7 kcat entries)

Substrate kinetics (BRENDA)

4 substrates with measured Km, best-characterized 4. Km ranges are aggregated across organisms/conditions.

SubstrateKm (mM)Measurements
D-GLUCOSE 6-PHOSPHATE0.12–76.3431
GLUCOSE 6-PHOSPHATE0.035–2.8513
NAD+0.0051–1.9513
2-DEOXY-D-GLUCOSE 6-PHOSPHATE1.151

Catalyzed reactions (Rhea), 1 shown:

  • D-glucose 6-phosphate = 1D-myo-inositol 3-phosphate (RHEA:10716)

UniProt features (46 total): binding site 25, sequence conflict 12, mutagenesis site 3, modified residue 2, splice variant 2, chain 1, region of interest 1

Structure

Experimental structures (PDB)

0 structures.

Predicted structure (AlphaFold)

ModelpLDDTFraction very-high
AF-Q9NPH2-F192.230.88

Functional residue map

Curated UniProt residues grouped by drug-discovery relevance — catalytic, ligand-binding, modification, and mutation-validated positions. Source: UniProtKB sequence features.

Ligand- & substrate-binding residues (25): 191; 228; 229; 230; 231; 278; 279; 303; 306; 337; 67; 338

Post-translational modifications (2): 279, 357

Mutagenesis-validated functional residues (3):

PositionPhenotype
177decreases activity.
279decreases activity.
357decreases activity.

Function

Pathways and Gene Ontology

Reactome pathways

3 pathways

IDPathway
R-HSA-1855183Synthesis of IP2, IP, and Ins in the cytosol
R-HSA-1430728Metabolism
R-HSA-1483249Inositol phosphate metabolism

MSigDB gene sets: 192 (showing top): GOBP_PHOSPHOLIPID_METABOLIC_PROCESS, MODULE_255, GOBP_POLYOL_METABOLIC_PROCESS, GRAESSMANN_APOPTOSIS_BY_DOXORUBICIN_UP, MODULE_317, GOBP_ORGANOPHOSPHATE_METABOLIC_PROCESS, GOBP_ORGANOPHOSPHATE_BIOSYNTHETIC_PROCESS, GOBP_PHOSPHOLIPID_BIOSYNTHETIC_PROCESS, GOBP_SMALL_MOLECULE_BIOSYNTHETIC_PROCESS, KIM_RESPONSE_TO_TSA_AND_DECITABINE_UP, SHETH_LIVER_CANCER_VS_TXNIP_LOSS_PAM3, MCLACHLAN_DENTAL_CARIES_DN, SMID_BREAST_CANCER_LUMINAL_B_UP, MORI_SMALL_PRE_BII_LYMPHOCYTE_DN, GOBP_INOSITOL_METABOLIC_PROCESS

GO Biological Process (3): inositol biosynthetic process (GO:0006021), phospholipid biosynthetic process (GO:0008654), lipid metabolic process (GO:0006629)

GO Molecular Function (3): inositol-3-phosphate synthase activity (GO:0004512), protein binding (GO:0005515), isomerase activity (GO:0016853)

GO Cellular Component (2): cytoplasm (GO:0005737), cytosol (GO:0005829)

Reactome top-level categories

Rollup of top-2 pathways:

CategoryPathways
Inositol phosphate metabolism1
Metabolism1

GO top-level categories

Rollup of top GO terms by namespace:

CategoryTerms
cellular anatomical structure2
inositol metabolic process1
polyol biosynthetic process1
phospholipid metabolic process1
lipid biosynthetic process1
organophosphate biosynthetic process1
primary metabolic process1
intramolecular lyase activity1
binding1
catalytic activity1
intracellular anatomical structure1
cytoplasm1

Protein interactions and networks

STRING

1782 interactions, top by confidence (×1000):

Protein AProtein BPartner UniProtScore
ISYNA1IMPA1P29218743
ISYNA1CDIPTO14735724
ISYNA1GTF2BQ00403707
ISYNA1MIOXQ9UGB7702
ISYNA1IMPA2O14732646
ISYNA1KAT2AQ92830635
ISYNA1SIK1P57059598
ISYNA1KAT2BQ92831589
ISYNA1IP6K1Q92551582
ISYNA1INPP1P49441575
ISYNA1HK1P19367569
ISYNA1ATP1A1P05023561
ISYNA1ATP1A4Q13733559
ISYNA1IPMKQ8NFU5553
ISYNA1CLCQ05315540

IntAct

49 interactions, top by confidence:

ABTypeScore
TRAF4ISYNA1psi-mi:“MI:0915”(physical association)0.720
ISYNA1TRAF4psi-mi:“MI:0915”(physical association)0.720
CFTRESYT2psi-mi:“MI:0914”(association)0.710
USE1NBASpsi-mi:“MI:0914”(association)0.640
ISYNA1CYSRT1psi-mi:“MI:0915”(physical association)0.560
CYSRT1ISYNA1psi-mi:“MI:0915”(physical association)0.560
POT1ISYNA1psi-mi:“MI:0915”(physical association)0.510
DUSP14ISYNA1psi-mi:“MI:0915”(physical association)0.370
HSPA8PPP6Cpsi-mi:“MI:0914”(association)0.350
PRNPWDR91psi-mi:“MI:0914”(association)0.350
TEX101PSMD12psi-mi:“MI:0914”(association)0.350
Mpsi-mi:“MI:0914”(association)0.350
CSAG1NAP1L4psi-mi:“MI:0914”(association)0.350
CSAG2CAMK2Dpsi-mi:“MI:0914”(association)0.350
MAP2K2IPO5psi-mi:“MI:0914”(association)0.350
IRAK1ILVBLpsi-mi:“MI:0914”(association)0.350
MYLKACOT7psi-mi:“MI:0914”(association)0.350
STK32CILVBLpsi-mi:“MI:0914”(association)0.350
ULK4NDUFA4psi-mi:“MI:0914”(association)0.350
NRBP2HAX1psi-mi:“MI:0914”(association)0.350
RPS6KL1GOLIM4psi-mi:“MI:0914”(association)0.350
MAPTSHTN1psi-mi:“MI:0914”(association)0.350

BioGRID (83): ISYNA1 (Two-hybrid), ISYNA1 (Affinity Capture-MS), ISYNA1 (Affinity Capture-MS), ACAA2 (Co-fractionation), ALDOA (Co-fractionation), ARHGDIA (Co-fractionation), ECHS1 (Co-fractionation), ERN2 (Co-fractionation), HNRNPH2 (Co-fractionation), ISYNA1 (Co-fractionation), ISYNA1 (Co-fractionation), ISYNA1 (Co-fractionation), ISYNA1 (Co-fractionation), ISYNA1 (Co-fractionation), ISYNA1 (Co-fractionation)

ESM2 similar proteins: A0A0A2JW88, A0A2C9EVE6, A0A2K9M484, A2YH64, A8C7R6, B0RP37, B1MK49, F7J5X9, J9VYL3, M1WA44, O09345, O84716, P17598, P18123, P25890, P48351, P48352, P49315, P49316, P49317, P49319, P55037, P55038, P55305, P55307, P55311, P55313, P78574, Q01297, Q08262, Q0D9C4, Q21FJ6, Q4K423, Q4R6E3, Q4UYF2, Q5QQ46, Q6AYK3, Q6DAM0, Q6ZXC2, Q715L4

Diamond homologs: C0HM53, J9VYL3, O64437, O65195, O97477, P11986, P42800, P42801, P42802, P42803, Q2NL29, Q38862, Q40271, Q41107, Q4R6E3, Q54N49, Q5QQ46, Q6AYK3, Q6DDT1, Q6FQI1, Q7ZXY0, Q8A7J8, Q8I3Y8, Q8S5N2, Q96348, Q9FPK7, Q9FYV1, Q9JHU9, Q9LW96, Q9LX12, Q9NPH2, Q9S7U0, Q9SSV4

SIGNOR signaling

2 interactions.

AEffectBMechanism
E2F1“up-regulates quantity by expression”ISYNA1“transcriptional regulation”
ISYNA1“up-regulates quantity”“1D-myo-inositol 1-phosphate”“chemical modification”

Disease & clinical

Clinical variants and AI predictions

ClinVar

93 variants total. Per-class counts are floors (≥ shown; pagination cap):

ClassificationCount (floor)
Pathogenic0
Likely pathogenic0
Uncertain significance65
Likely benign6
Benign5

Top pathogenic / likely-pathogenic (0)

SpliceAI

1479 predictions. Top by Δscore:

VariantEffectΔscore
19:18435211:G:Cdonor_gain1.0000
19:18435215:AG:Adonor_gain1.0000
19:18435216:G:Cdonor_gain1.0000
19:18435312:C:CTdonor_gain1.0000
19:18435313:C:CTdonor_gain1.0000
19:18435567:CA:Cdonor_gain1.0000
19:18435756:CG:Cdonor_gain1.0000
19:18435839:T:TAdonor_gain1.0000
19:18435917:ATGGT:Aacceptor_gain1.0000
19:18435918:TGGT:Tacceptor_gain1.0000
19:18435919:GGT:Gacceptor_gain1.0000
19:18435920:GT:Gacceptor_gain1.0000
19:18435921:TCTG:Tacceptor_loss1.0000
19:18435922:C:CCacceptor_gain1.0000
19:18435922:C:CGacceptor_loss1.0000
19:18435923:T:Aacceptor_loss1.0000
19:18436028:GCACC:Gdonor_loss1.0000
19:18436029:CA:Cdonor_loss1.0000
19:18436073:TG:Tdonor_gain1.0000
19:18436243:CCGAG:Cacceptor_gain1.0000
19:18436244:CGAG:Cacceptor_gain1.0000
19:18436244:CGAGC:Cacceptor_gain1.0000
19:18436245:GAG:Gacceptor_gain1.0000
19:18436245:GAGC:Gacceptor_loss1.0000
19:18436246:AG:Aacceptor_gain1.0000
19:18436247:GCTG:Gacceptor_loss1.0000
19:18436248:C:CCacceptor_gain1.0000
19:18436248:C:Tacceptor_loss1.0000
19:18436325:CCCA:Cdonor_loss1.0000
19:18436326:CCA:Cdonor_loss1.0000

AlphaMissense

3620 scored. Top likely-pathogenic:

VariantProtein changeam_pathogenicity
19:18435481:G:CD419E0.999
19:18435481:G:TD419E0.999
19:18435482:T:AD419V0.999
19:18435482:T:GD419A0.999
19:18435483:C:GD419H0.999
19:18435638:C:AK393N0.999
19:18435638:C:GK393N0.999
19:18435343:C:AK465N0.998
19:18435343:C:GK465N0.998
19:18435482:T:CD419G0.998
19:18435841:C:AK352N0.998
19:18435841:C:GK352N0.998
19:18435886:G:CN337K0.998
19:18435886:G:TN337K0.998
19:18435890:C:AG336V0.998
19:18435898:G:CN333K0.998
19:18435898:G:TN333K0.998
19:18435904:A:CS331R0.998
19:18435904:A:TS331R0.998
19:18435906:T:GS331R0.998
19:18436092:C:AK305N0.998
19:18436092:C:GK305N0.998
19:18436098:G:CD303E0.998
19:18436098:G:TD303E0.998
19:18436099:T:AD303V0.998
19:18436099:T:GD303A0.998
19:18436875:A:GW140R0.998
19:18436875:A:TW140R0.998
19:18436071:C:AK312N0.997
19:18436071:C:GK312N0.997

dbSNP variants (sampled 300 via entrez): RS1000104116 (19:18438462 T>TC), RS1000539586 (19:18438576 C>T), RS1001141260 (19:18437319 C>G,T), RS1001336449 (19:18439095 TG>T), RS1001719078 (19:18435826 G>T), RS1001880132 (19:18434001 C>A,G,T), RS1002340539 (19:18438127 A>G,T), RS1002787875 (19:18434452 C>G,T), RS1003125239 (19:18434621 G>A,C), RS1003258871 (19:18435019 G>A), RS1003959546 (19:18434105 T>C,G), RS1004385571 (19:18438387 T>TTACAAGAGG), RS1005037936 (19:18438642 G>A,T), RS1005090335 (19:18438355 C>A,T), RS1005648712 (19:18439340 G>A,C)

Disease associations

OMIM: gene MIM:611670 | disease phenotypes:

GenCC curated gene-disease

Mondo (0):

Orphanet (0):

HPO phenotypes

0 total (0 of 0 shown, HPO-id order):

GWAS associations

3 associations (top):

StudyTraitp-value
GCST001937_26Breast cancer5.000000e-15
GCST006979_750Heel bone mineral density1.000000e-15
GCST007511_22Alzheimer’s disease (late onset)2.000000e-06

EFO canonical traits (2, from GWAS)

EFO IDTrait name
EFO:0009270heel bone mineral density
EFO:1001870late-onset Alzheimers disease

Drugs & pharmacology

Drug and pharmacology data

Is drug target: no

PharmGKB: 1 entry (VIP=true, CPIC=false)

CTD chemical–gene interactions

69 total (human), top 30 by PubMed support.

ChemicalActions (top 5)PubMed papers
Benzo(a)pyreneaffects methylation, decreases methylation, increases expression5
Air Pollutantsaffects expression, increases abundance, decreases expression, increases expression4
Estradiolaffects expression, affects cotreatment, decreases expression4
Aflatoxin B1affects expression, decreases methylation, increases expression4
sodium arseniteaffects binding, increases reaction, affects cotreatment, decreases expression, increases abundance (+1 more)3
bisphenol Aaffects expression, increases expression2
Nickeldecreases expression, increases expression2
Smokedecreases expression, increases abundance, increases expression2
Tobacco Smoke Pollutionaffects expression, decreases expression2
Valproic Acidincreases expression, increases methylation2
Cyclosporinedecreases expression2
Particulate Matterdecreases expression, increases abundance, increases expression2
aristolochic acid Iincreases expression1
bisphenol Faffects cotreatment, increases expression1
2,4,6-tribromophenolincreases expression1
triphenyl phosphateaffects expression1
sodium arsenatedecreases expression1
decabromobiphenyl etherincreases expression1
mono-(2-ethylhexyl)phthalateincreases expression1
cobaltous chloridedecreases expression1
manganese chlorideaffects cotreatment, decreases expression, increases abundance1
2,3-bis(3’-hydroxybenzyl)butyrolactoneaffects cotreatment, increases expression1
nickel sulfatedecreases expression1
entinostatincreases expression1
4-(5-benzo(1,3)dioxol-5-yl-4-pyridin-2-yl-1H-imidazol-2-yl)benzamideaffects cotreatment, increases expression1
nutlin 3affects cotreatment, increases secretion1
2,2’,4,4’-tetrabromodiphenyl etherincreases expression1
pentabrominated diphenyl ether 100increases expression1
hexabrominated diphenyl ether 153increases expression1
bisphenol Sdecreases expression1

Clinical trials (associated diseases)

0 trials via MONDO — disease-level, not drug-specific.

No linked Atlas pages yet — the cross-entity mesh grows as the corpus expands.