The c.878T>C variant in PAH is a missense variant predicted to cause substitution of leucine by serine at amino acid 293 (p.Leu293Ser). At least one patient with this variant displayed plasma phenylalanine concentration persistently above 120umol/L (2mg/dL) AND sequencing of genes in the BH4 cofactor metabolism pathway excluded a defect of BH4 cofactor metabolism, which is highly specific for PAH deficiency (PP4_Moderate, PMID: 21147011). This variant is absent from gnomAD v4.1.0 (PM2_Supporting). The computational predictor REVEL gives a score of 0.968, which meets the threshold of 0.932, evidence that correlates with strong impact to PAH function (PP3_Strong). Another missense variant c.877T>A (p.Leu293Met) [CA16020883] in the same codon has been classified as likely pathogenic for PAH deficiency by the ClinGen PAH Variant Curation Expert Panel (PM5_Supporting). In summary, this variant meets the criteria to be classified as likely pathogenic for autosomal recessive PAH deficiency based on the ACMG/AMP criteria applied, as specified by the ClinGen PAH Variant Curation Expert Panel: PP3_strong, PP4_moderate, PM2_supporting, PM5_supporting (PAH VCEP specifications version 2.0.0; approved July 16, 2024).