The NM_000203.5:c.46_57del variant results in an inframe deletion of 4 amino acids (p.Ser16_Ala19del) within the lysosomal signal sequence of IDUA (PM4). This variant is named "134del12" in older literature. At least twelve patients with with variant and a diagnosis of mucopolysaccharidosis type 1 have been reported in the literature. This includes four patients with documented laboratory values showing deficiency of IDUA activity in fibroblasts or leukocytes (PMID: 15300847, 21394825, 23786846) one of whom also had documented clinical features consistent with the diagnosis including corneal clouding, joint stiffness, digital contractures, cardiac valve disease, airway obstruction, and developmental delay (PMID: 15300847) (PP4). Four individuals have been reported who are compound heterozygous for the variant and another variant in IDUA that has been classified as pathogenic by the ClinGen Lysosomal Diseases VCEP including two confirmed in trans - c.1750C>T (p.Gln584Ter) (PMID: 12189649) and p.Trp402Ter (PMID: 11735025); and two phase unknown - c.208C>T (p.Gln70Ter) (PMID: 21394825) and c.386-2A>G (PMID: 11735025). In addition, at least two homozygotes have been reported PMID: 7951228, 21394825) (PM3_VeryStrong). Additional patients are compound heterozygous for the variant and c.603C>G (p.Tyr201Ter) (PMID: 21394825), c.1189+5G>A (PMID: 21394825); c.1960T>C (p.Ter654ArgextTer*62) (PMID: 21394825), c.1049A>T (p.Asn350Ile) (PMID: 12559846) and c.1598C>G (p.Pro533Arg) (PMID: 15300847). The allelic data from these patients will be used in the classification of the second variant and is not included here to avoid circular logic. When expressed in COS-7 cells, the variant (labeled as "134del12") resulted in 124.6% of normal activity. A 77-kDa precursor protein was observed on Western blot, compared with a major mature 63-kDa form and a minor 77-kDa precursor in cells transfected with wild-type cDNA. These results suggested that the variant prevents correct posttranslational processing and transport to the lysosome (PMID: 12189649) (PS3_Supporting). The highest population minor allele frequency in gnomAD v4.1.0 is 0.00006251 (69/1103836 alleles) in the European non-Finnish population, which is lower than the ClinGen Lysosomal Diseases VCEP’s threshold for PM2_Supporting (<0.00025), meeting this criterion (PM2_Supporting). There is a ClinVar entry for this variant (Variation ID: 92643). 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): PM3_Very Strong, PM4, PP4, PS3_Supporting, PM2_Supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel on December 6, 2024)