The NM_000152.5: c.1979G>A variant in GAA is a missense variant predicted to cause substitution of arginine by histidine at amino acid 660 (p.Arg660His). More than nine individuals with features consistent with Pompe disease have been reported with this variant. Of these patients 3 probands (and two siblings) have documented laboratory values indicating GAA deficiency in the affected range in dried blood spot, or muscle, or <1% activity in cultured skin fibroblasts (PMID: 21484825, 25037089, 27649523), and three probands are reported to have low GAA activity and to be on enzyme replacement therapy (PP4_Moderate). Six patients are compound heterozygous for the variant and a variant classified as pathogenic by the ClinGen LSD VCEP. For two patients, the second variant was confirmed in trans by parental testing - c.525delT (PMID: 30559630, possibly same case in PMID: 30214072), and c.670C>T (p.Arg224Trp) (PMID: 14643388) – and in four patients the phase of the second variant is unconfirmed - c.655G>A (p.Gly219Arg) (PMID: 29122469), c.1082C>T (p.Pro361Leu) (PMID: 21484825), c.1998C>T (p.Arg600Cys) (PMID: 14643388), and c.2560C>T (p.Arg854Ter) (PMID: 30214072). Two homozygous siblings have also been reported (PMID: 27649523) (PM3_Very Strong). Two probands have been reported who are compound heterozygous for the variant and either c.-32-17_-32-10del8ins30 (PMID: 25037089; two affected siblings), or c.1114C>T (p.His372Tyr) (PMID: 30214072). The allelic data from these patients will be used in the classification of the second variant and is not included here to avoid circular logic. The highest population minor allele frequency in gnomAD v2.1.1 is 0.00025 (6/23566) in the African population, which is lower than the ClinGen LSD VCEP’s threshold for PM2_Supporting (<0.001), meeting this criterion (PM2_Supporting). In two independent studies, in which the variant was expressed in COS cells, the GAA activity was <2% wild type, indicating that the variant has a damaging effect on GAA function (PS3_Supporting). The computational predictor REVEL gives a score of 0.976 which is above the threshold of 0.7, evidence that correlates with impact to GAA function (PP3). Another variant, c.1978C>T (p.Arg660Cys), has been reported at this position. p.Arg660His will be used to support the classification of p.Arg660Cys and thus PM5 is not applied here, in order to avoid circular logic. There is a ClinVar entry for this variant (Variation ID: 189172; 2 star review status) with nine submitters classifying the variant as pathogenic, and three as likely pathogenic. In summary, this variant meets the criteria to be classified as Pathogenic for Pompe disease. GAA-specific ACMG/AMP criteria met, based on the specifications of the ClinGen LSD VCEP (Specifications Version 2.0): PM3_VeryStrong, PP4_Moderate, PP3, PS3_Supporting, PM2_Supporting. (Approved by LSD VCEP on Feb 15, 2022).