The NM_000152.5:c.510C>T variant in GAA is a synonymous (silent) variant (p.Asp170=) that is not predicted to impact splicing (BP7, BP4). This variant has been detected in at least 14 individuals with Pompe disease (PMIDs 14695532, 30564623, 30922962, 34405923). However, in all but one individual it was found to co-occur with two additional GAA variants. Additionally, studies have shown that c.510C>T frequently occurs in cis with the c.-32-13T>G variant (pathogenic by the ClinGen Lysosomal Disease VCEP) and is associated with an earlier age of onset and lower GAA enzyme activity in fibroblasts compared to individuals with c.-32-13T>G that do not carry c.510C>T (PMIDs 30922962, 34405923). RT-PCR from patients with c.510C>T and c.-32-13T>G and minigene assays demonstrate that c.510C>T modulates the aberrant splicing caused by c.-32-13T>G, but does not have a significant impact on splicing when it occurs in isolation (PMID 30922962). The highest population minor allele frequency in gnomAD v2.1.1. is 0.00008925 (11/123248 alleles) in the European (non-Finnish) population, which is lower than the ClinGen Lysosomal Diseases VCEP’s threshold for PM2_Supporting (<0.001), meeting this criterion (PM2_Supporting). There is a ClinVar entry for this variant (Variation ID: 280956). While it is uncertain if c.510C>T is causative of disease, there is evidence suggesting that this variant acts as a negative modifying factor when it occurs in cis with c.-32-13T>G. Therefore, this variant is classified as a variant of unknown significance for Pompe disease based on the ACMG/AMP criteria applied, as specified by the ClinGen Lysosomal Diseases Variant Curation Expert Panel (Specifications Version 2.0): BP4, BP7, PM2_Supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel on March 19, 2024).