The NM_000203.5:c.1091C>T variant in IDUA is a missense variant predicted to cause substitution of threonine by methionine at amino acid 364 (p.Thr364Met). This variant has been identified in at least 6 probands with MPS I. Three individuals are compound heterozygous for the variant and another variant in IDUA that has been classified as pathogenic for MPS I by the ClinGen LD VCEP, c.613_617dup (p.Glu207fs) (ClinVar Variation ID: 11921); phase unconfirmed (PMID: 15521993) (max 2 x 0.5 points = 1 point). Two individuals were apparently homozygous for the variant (PMID: 9391892, 16435211; 2 x 0.5 points; one of them is possibly compound heterozygous for the variant and a gene deletion) (PMID: 9391892). Another individual is compound heterozygous for the variant and c.589G>A p.(Gly197Ser) (PMID: 33301762). The allelic data from this patient will be used in the assessment of p.Gly197Ser and is not included here to avoid circular logic. Total 2 points (PM3_Strong). Patients reported with this variant had features specific to MPS I, including one patient with clinical symptoms consistent with the diagnosis, documented IDUA deficiency, and elevated urine GAGs that reduced upon treatment with ERT (PMID: 16435211) (PP4_Moderate). The highest population minor allele frequency in gnomAD v4.1.0 is 0.00002245 (1/44544 alleles) in the East Asian population, which is lower than the ClinGen Lysosomal Diseases VCEP’s threshold for PM2_Supporting (<0.00025), meeting this criterion (PM2_Supporting). Expression of the variant in COS-7 cells resulted in ~1% wild type IDUA activity and evidence of abnormal IDUA synthesis and processing indicating that this variant may impact protein function (PMIDs: 9391892, 10466419; PS3_Supporting). The computational predictor REVEL gives a score of 0.908 which is above the threshold of 0.773, evidence that correlates with impact to IDUA function at the moderate level based on the specifications of the ClinGen Lysosomal Diseases VCEP (PMID: 36413997; PP3_Moderate). There is a ClinVar entry for this variant (Variation ID: : 11925). In summary, this variant meets the criteria to be classified as pathogenic for MPS I based on the ACMG/AMP criteria applied, as specified by the ClinGen Lysosomal Diseases Variant Curation Expert Panel (Specifications Version 1.0.0): PM3_Strong, PP3_Moderate, PP4_Moderate,, PM2_Supporting, PS3_Supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel, May 16, 2025).