The NM_000152.5:c.1556T>C variant in GAA is a missense variant predicted to cause substitution of methionine by threonine at amino acid 519 (p.Met519Thr). This variant has been detected in at least 4 individuals with Pompe disease. Of those individuals, 3 were compound heterozygous for the variant and a variant that has been classified as pathogenic by the ClinGen LD VCEP, phase unconfirmed, either c.525del (ClinVar Variation ID: 4033, SCV001443331.1) (PMID: 14695532), c.1441T>C (p.Trp481Arg) (ClinVar Variation ID: 189007, SCV004227917.1) (PMID: 31086307), or c.[752C>T;761C>T] (p.[Ser251Leu;Ser254Leu] (ClinVar Variation ID 325781/325782) (Clinical laboratory data); One individual was homozygous for the variant (PMID: 33741225) (PM3_Strong). At least 1 patient with this variant had documented GAA deficiency with <10% of normal mean control level of GAA activity in dried blood spot (PMID: 33741225) and 2 patients were reported to have symptoms consistent with infantile onset Pompe disease (PMID: 14695532) (PP4_Moderate). The highest population minor allele frequency in gnomAD v2.1.1 is 0.00003 (1/30616 alleles) in the South Asian population, which is lower than the ClinGen Lysosomal Diseases VCEP’s threshold for PM2_Supporting (<0.001), meeting this criterion (PM2_Supporting). The computational predictor REVEL gives a score of 0.983 which is above the threshold of 0.7, evidence that correlates with impact to GAA function (PP3). Expression of the variant in COS-7 resulted in <2% wild type GAA activity with western blot data showing decreased level of mature protein, indicating that this variant may impact protein function (PMIDs: 19862843, 14695532) (PS3_Moderate). This variant alters amino acid M519, a residue that crystallography studies have shown to be important in the active site of GAA, and therefore has been defined as a critical residue by the ClinGen Lysosomal Diseases VCEP (https://www.biorxiv.org/content/10.1101/212837v1.full.pdf, PMID: 29061980) (PM1). There is a ClinVar entry for this variant (Variation ID: 189124). In summary, this variant meets the criteria to be classified as Pathogenic for Pompe disease. GAA-specific ACMG/AMP criteria met, based on the specification of the ClinGen Lysosomal Diseases VCEP (Specifications Version 2.0): PM3_strong, PP4_moderate, PM1, PS3_moderate, PP3, PM2_supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel on May 7, 2024)