The NM_000152.5:c.1585_1586delinsGT variant in GAA is predicted to cause substitution of serine by valine at amino acid 529 (p.Ser529Val). At least 5 patients with this variant had documented GAA deficiency with <10% of normal mean control level of GAA activity in leukocytes, lymphocytes, muscle samples or were noted to have deficient GAA activity but results were not provided(PMIDs: 11053688, 17805474, 21984055) (PP4_Moderate). Of those individuals, 2 were compound heterozygous for the variant and a pathogenic variant as classified by the ClinGen Lysosomal Diseases VCEP (PMIDs 17805474, 11053688). Three individuals were homozygous for the variant (PMIDs: 21984055, 11053688) (PM3_Strong). This variant is absent in gnomAD v4.1.0. (PM2_Supporting). This variant results in 14.1% GAA activity when expressed in COS-1 cells by Tsunoda et al, 1996 (PMID 8834250), 10% GAA activity when expressed in SV40 immortalized fibroblasts (TR4912) by Tsujino S et al, 2000 (PMID 11053688), 9.0% wild-type activity in transiently transfected HEK293T cells by Shimada Y et al, 2014 (PMID 25256446), and 14% in transfected COS-7 cells by Flanagan et al, 2009 (PMID: 19862843) (PS3_Supporting). There is a ClinVar entry for this variant (Variation ID: 4026, 0 star review status). In summary, this variant meets the criteria to be classified as Likely Pathogenic for Pompe disease based on the GAA-specific ACMG/AMP criteria applied, as specified by the ClinGen Lysosomal Diseases Variant Curation Expert panel (Specifications Version 2.0): PM3_Strong, PP4_Moderate, PM2_Supporting, PS3_Supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel on June 17, 2025)