The NM_000277.3(PAH):c.361T>C variant in PAH is a missense variant predicted to cause substitution of phenylalanine by leucine at amino acid 121 (p.Phe121Leu). 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 to exclude a defect of BH4 cofactor metabolism, which is highly specific for PAH deficiency (PP4_Moderate, PMIDs: 28754886, 30459323). This variant has been detected in at least 2 individuals with Phenylketonuria. Both were compound heterozygous for the variant and a pathogenic variant (p.Arg241Cys). Parental testing was not reported. (PMIDs: 30459323, 28754886) (PM3). This variant is absent from gnomAD v4.1.0 (PM2_Supporting). In vitro expression study of this variant in COS-7 cells showed <50% enzyme activity in vitro with < 10 benign/pathogenic variant controls, indicating that this variant impacts protein function (PMID: 24327145; PS3_Supporting). The computational predictor REVEL gives a score of 0.995, which meets the threshold of 0.932, evidence that correlates with strong impact to PAH function (PP3_Strong). In summary, this variant meets criteria to be classified as pathogenic for PAH deficiency. PAH-specific ACMG/AMP criteria applied: PS3_supporting, PM2_supporting, PM3, PP3_strong, PP4_moderate. Version 2.0, 7/16/2024.