The NM_000203.5:c.1743C>G (p.Tyr581Ter) variant in IDUA is a nonsense variant predicted to cause a premature stop codon in biologically-relevant-exon 13 out of 14. While the premature stop codon is predicted to occur in the penultimate exon of the gene, it is 5' to the last 50 nucleotides of the exon and, therefore, this variant is expected to lead to nonsense mediated decay in a gene in which loss-of-function is an established disease mechanism (PVS1). At least 5 patients with this variant and a diagnosis of MPS I, confirmed by reduced IDUA activity, have been reported. This included patients with documented laboratory values indicating undetectable IDUA activity along with clinical features such as intellectual disability, dysmorphic facial features, multiple dysostosis, joint stiffness, hepatosplenomegaly, umbilical hernia, chronic rhinorrhea, and hydrocephalus (PMID: 22074387, 27196898) (PP4). One patient is compound heterozygous for the variant and another variant in IDUA that has been classified as pathogenic by the ClinGen Lysosomal Diseases VCEP, c.1205G>A (p.Trp402Ter) (ClinVar Variation ID: 11908, PMID: 3351789); the phase is unknown (0.5 points). Two Tunisian patients are homozygous for the variant (PMID: 22074387) (2 x 0.5 points). A further two patients are compound heterozygous for the variant and p.Pro533Arg (PMID: 12796790, 27196898); the allelic data from these patients will be used in the classification of p.Pro533Arg and are not included here to avoid circular logic. Total 1.5 points (PM3). The highest population minor allele frequency in gnomAD v4.1.0. is 0.00008921 (4/44840 alleles) in the East Asian population, which is lower than the ClinGen Lysosomal Diseases VCEP’s threshold for PM2_Supporting (<0.0025), meeting this criterion (PM2_Supporting). In summary, this variant meets the criteria to be classified as pathogenic for mucopolysaccharidosis type 1. IDUA-specific ACMG/AMP criteria applied, as specified by the ClinGen Lysosomal Diseases VCEP (Specifications Version 1.0.0.): PVS1, PM3, PP4, PM2_Supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel on December 5, 2024)