The NM_000203.5: c.156C>A variant in IDUA is a missense variant predicted to cause substitution of phenylalanine by Leucine at amino acid 52 (p.Phe52Leu). This variant has been detected in at least one individual with MPS I. This individual is compound heterozygous for the variant and c.1037T>G p.(Leu346Arg), which is classified as likely pathogenic by the ClinGen Lysosomal Diseases VCEP (PMID 21480867, phase confirmed in trans, 1 point) (PM3). This individual has documented IDUA deficiency within the affected range (0.2 nmol/h/mg, reference range 27.2-52), increased excretion of urinary dermatan sulfate and heparin sulfate, and clinical features specific to MPS I including hepatosplenomegaly, joint stiffness and airway obstruction (PMID 21480867) (PP4_moderate). This variant is absent in gnomAD v4.1.0. (PM2_Supporting). The computational predictor REVEL gives a score of 0.669 which is in the range 0.644-0.773, evidence that correlates with impact to IDUA function at the supporting level (PMID: 36413997) (PP3_supporting). Another variant resulting in the same missense change (c.156C>G, p.Phe52Leu) is in ClinVar (Variation ID: 554280) with a low frequency in gnomAD; reported in Abdulelah et al, Iraqi J Pharm Sci, Vol.33(4) 2024). This variant has not yet been classified by the ClinGen LD VCEP. There is no ClinVar entry for this variant. In summary, this variant meets the criteria to be classified as likely pathogenic for MPS I based on the IDUA-specific ACMG/AMP criteria applied, as specified by the ClinGen Lysosomal Diseases Variant Curation Expert panel (Specifications Version 1.0.0.): PM3, PP4_moderate, PP3, PM2_supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel on April 7, 2025)