The NM_000152.5:c.953T>A variant in GAA is a missense variant predicted to cause substitution of methionine by lysine at amino acid 318 (p.Met318Lys). This variant is absent from gnomAD v2.1.1 (PM2_Supporting). This variant was found in compound heterozygosity (phase unknown) with a unique pathogenic variant in GAA in one patient with Pompe disease (PMID: 21484825). Additional cases (PMID: 18425781) have been reported but were not included because a second variant and/or zygosity was not specified (PP4_Moderate, PM3_Supporting). Expression of the variant in COS cells resulted in 3.2% wild type GAA activity and evidence of abnormal GAA synthesis and processing, leading the variant to be described as Class B (“potentially less severe”), indicating that this variant may impact protein function (PMID 22644586)(PS3_Moderate). The computational predictor REVEL gives a score of 0.926 which is above the threshold of 0.7, evidence that correlates with impact to GAA function (PP3). There is a ClinVar entry for this variant (Variation ID: 558700; 1 star review status) with 1 submitter classifying the variant as likely pathogenic and 1 submitter classifying the variant as a VUS. In summary, this variant meets the criteria to be classified as likely pathogenic for Pompe disease based on the ACMG/AMP criteria applied, as specified by the ClinGen Lysosomal Storage Disorders Variant Curation Expert panel (Specifications Version 2.0): PP4_Moderate, PS3_Moderate, PP3, PM2_Supporting, PM3_Supporting. (Classification approved by the ClinGen LSD VCEP - Oct. 19, 2021).