The NM_000203.4:c.536C>G variant in IDUA is a missense variant predicted to cause substitution of Threonine by Arginine at amino acid 179 (p.Thr179Arg). This variant has been reported in at least eight individuals with MPS I (PMID: 35433540, 21480867, 29095814, 34813777). This variant has been reported in at least eight individuals with MPS I (PMID: 35433540, 21480867, 29095814, 34813777): one is homozygous for this variant (PMID: 21480867, 0.5 points), six are compound heterozygous for this variant and a second variant that has been classified as pathogenic or likely pathogenic by the ClinGen Lysosomal Diseases VCEP including c.877G>A p.(Trp626Ter) (PMID: 35433540, phase confirmed, 1 point), c.236C>T p.(Ala79Val) (PMID: 21480867, 2 patients, phase unconfirmed, 2 x 0.5 points), c.1898C>T p.(Ser633Leu) (PMID: 21480867, 2 patients, phase unconfirmed, 2 x 0.5 points), c.1601C>A p.(Ser534Ter) (PMID: 34813777, phase not confirmed, 0.5 point). One additional patient had a clinical diagnosis and carried this variant but a second variant was not detected. 4 points (PM3_VeryStrong). One patient with this variant present in the homozygous state has been reported with a clinical diagnosis of Hurler syndrome, presenting with delayed development, macrocephaly, congenital dermal melanocytosis, and hepatosplenomegaly; this patient also had reduced IDUA enzyme activity (0.1 nmol/h/mg, reference range 27.2-52) and increased excretion of urinary dermatan sulfate and heparan sulfate (PMID: 21480867) (PP4_moderate). The population minor allele frequency in gnomAD v4.1.0 is 8.5e-7 (1/1,179,952 alleles), which is lower than the ClinGen Lysosomal Diseases VCEP’s threshold for PM2_Supporting (<0.00025), meeting this criterion (PM2_Supporting). The computational predictor REVEL gives a score of 0.927 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). Other missense variants at the same position have been reported in individuals with MPS1, including c.536C>T p.(Thr179Met) (PMID: 32670797, ClinVar ID: 1455223), and p.Thr179Lys (PMID: 28752568, ClinVar ID: 1455223). The classification of c.536C>G (p.Thr179Arg) will be used in the assessment of the other variants at Thr179 and is not included here to avoid circular logic. There is a ClinVar entry for this variant (Variation ID: 556358). 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_very strong; PP3_moderate; PP4_moderate; PM2_supporting (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel on December 5, 2024)