The NM_000152.5:c.1669A>T variant in GAA is a missense variant predicted to result in substitution of isoleucine by phenylalanine at amino acid 557 (p.Ile557Phe). The variant has been reported to be in cis, or phase unconfirmed, with c.2132C>G (p.Thr711Arg) in some cases. Excluding those cases, five probands in whom the p.Ile557Phe variant is in compound heterozygosity with a variant other than p.Thr711Arg, have been reported, all with infantile onset Pompe disease. At least one of these patients has documented features of infantile onset Pompe disease and deficient GAA activity (PMID: 36428004, 38162137, Clinical Laboratory) (PP4_Moderate). Five patients with IOPD have been reported who are compound heterozygous for the variant and another variant in GAA that has been classified as pathogenic or likely pathogenic for Pompe disease by the ClinGen LD VCEP. For four of these patients, the second variant was confirmed to be in trans; the second variant was either c.1935C>A (p.Asp645Glu) (P, ClinVar Variation ID: 4029, 1 point) (PMID: 38162137), c.2662G>T (p.Glu888Ter) (P, ClinVar ID Variation ID: 578595, 1 point) (PMID: 38162137), or c.1978C>T (p.Arg660Cys) (P, ClinVar Variation ID: 558604, 1 point) (Duke) or c.1843G>A (p.Gly615Arg) (LP, ClinVar Variation ID: 188786) (PMID: 36428004). Another patient was compound heterozygous for the variant, phase unconfirmed, and c.2237G>A (p.Trp746Ter) (ClinVar Variation ID: 280063, 0.5 points) (PMID: 38162137), Total 4.5 points (PM3_Very_Strong). In addition, a patient with IOPD was compound heterozygous for c.118C>T (p.Arg40Ter) (paternal), and c.[1669A>T; 2132C>G] (p.[Ile557Phe:Thr711Arg]) (maternal). c.118C>T (p.Arg40Ter) (ClinVar Variation ID: 426593) is pathogenic based on classification by the ClinGen LD VCEP, while another is compound heterozygous for c.2799+4 A>G (LP based on classification by the ClinGen LD VCEP), c.2132C>G (p.Thr711Arg), and c.1669A>T (p.Ile557Phe), phase unknown. Due to the presence of three variants, phase unknown, this data was not included. Additional patients are compound heterozygous for the variant and c.1385T>C (p.Leu462Pro) (PMID: 25612604), c.1013A>T (p.Asp338Val) (PMID: 34134972), or c.2132C>G (p.Thr711Arg) (PMID: 23884227, 39010129). The allelic data from these patients will be used for the classification of the second variant and is not included here to avoid circular logic. The highest population minor allele frequency in gnomAD v4.1.0. is 0.00002228 (1/44880 alleles) in the East Asian population, which is lower than the ClinGen Lysosomal Diseases VCEP’s threshold for PM2_Supporting (<0.001), meeting this criterion (PM2_Supporting). Expression of the variant in COS7 or HEK293 cells resulted in 2.7% GAA activity in cells and 0.3% in medium, and evidence of abnormal synthesis and processing on Western blot, indicating that this variant may impact protein function (PMID: 22644586) (PS3_Moderate). The computational predictor REVEL gives a score of 0.662 which is neither above nor below the thresholds predicting a damaging (>0.7) or benign (<0.5) impact on GAA function. Two other missense variants, c.1669A>G (p.Ile557Val) (ClinVar Variation ID: 856959) and c.1670T>G (p.Ile557Ser) (PMID: 26474166), in the same codon have been reported in patients with Pompe disease; p.Ile557Ser (ClinVar Variation ID: 1693547) has been classified as likely pathogenic for Pompe disease by the ClinGen LD VCEP (PM5_Supporting). There is a ClinVar entry for this variant (Variation ID: 558634). 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 Lysosomal Diseases Variant Curation Expert Panel (Specifications Version 2.0.0): PM3_Very Strong, PS3_Moderate, PP4_Moderate, PM2_Supporting, PM5_Supporting. (Classification approved by the ClinGen Lysosomal Diseases Variant Curation Expert Panel, December 16, 2025).