The NM_000152.5:c.1551+1G>A variant alters the canonical donor splice site of intron 10 of GAA. RT-PCR and sequencing using cells from an individual who is compound heterozygous for this variant show that the predominant effect is skipping of exon 10 (GAA has 20 exons). Additionally, an immunoblot from a patient homozygous for this variant showed a specific band at ~105.8 kD, which is consistent with the size of the 110 kD precursor GAA protein minus 4.2 kD, caused by an in-frame skip of exon 10 (PMID: 25243733, 29428273). Skipping of exon 10 results in an in-frame deletion of amino acids 480-517. This region forms part of the GAA catalytic barrel (amino acids 347-727) including two residues, Trp481 and Trp516, which are important in active site architecture and substrate binding (Kroos et al, 2012, PMID 22253258; Deming et al, 2017; DOI 10.1101/212837; Roig-Zamboni et al, 2017, PMID: 29061980). Therefore, loss of this exon is expected to abolish GAA activity (PVS1). At least 5 patients with findings consistent with Pompe disease have been reported with this variant. GAA activity was reported for 5 patients and was deficient (<10% residual activity) (PMID: 29428273, 21550241, 27189384, 37542277) (PP4_Moderate). At least one patient is homozygous for the variant, 0.5 points (PMID: 29428273). One patient is compound heterozygous for the variant and c.1561G>A, p.E521K (PMID: 37542277, pathogenic by ClinGen LD VCEP, phase unknown), 0.5 points. Two patients are compound heterozygous for the variant and GAA variant c.-32-3C>G (PMID: 27189384). The allelic data from these patients will be used in the assessment of the second variant and is not included here to avoid circular logic. 1 point (PM3). This variant is absent in gnomAD v2.1.1 and gnomAD v4.1.0 (PM2_Supporting). Other variants at the same canonical donor splice site have been reported in patients with Pompe disease, including c.1551+1G>C, c.1551+1G>T, and c.1551+2T>G (www.pompevariantdatabase.nl). One of these variants, c.1551+1G>C, has been classified as Pathogenic by the ClinGen LD VCEP. There is a ClinVar entry for this variant (Variation ID: 1065143). In summary, this variant meets the criteria to be classified as pathogenic for Pompe disease. GAA-specific ACMG/AMP criteria met, as specified by the ClinGen Lysosomal Diseases Variant Curation Expert Panel (Specifications Version 2.0): PVS1, PM3, PP4_Moderate, PM2_Supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel on January 21, 2025)