The c.261C>A variant in PAH is a missense variant predicted to cause substitution of serine by arginine at amino acid 87 (p.Ser87Arg). At least one patient with this variant displayed plasma phenylalanine concentration persistently above 120umol/L (2mg/dL) AND normal urine pterins and normal DHPR activity, which is highly specific for PAH deficiency (PP4_Moderate, PMID: 15464430). This variant has been detected in at least 7 individuals with PAH deficiency who were compound heterozygous for the variant and distinct pathogenic or likely pathogenic variants and 2 of those were confirmed in trans by parental testing (4.0 PM3 points, PMID: 8088845, 10495930, 15464430, 23514811, 26666653, 31623983; PM3_Very-strong). The allele frequency in gnomAD v4.1.0 is 0.0001159, which is lower than the ClinGen PAH Variant Curation Expert Panel threshold (<0.0002) for PM2_Supporting, meeting this criterion (PM2_Supporting). Expression of this variant in a mammalian transcription-translation system showed <50% enzyme activity in vitro with < 10 benign/pathogenic variant controls indicating that this variant impacts protein function (PMID:11161839)(PS3_supporting). The computational predictor REVEL gives a score of 0.633, which is neither above nor below the thresholds predicting a damaging or benign impact on PAH function. In summary, this variant meets the criteria to be classified as Pathogenic for autosomal recessive PAH deficiency based on the ACMG/AMP criteria applied, as specified by the ClinGen PAH Variant Curation Expert Panel: PP4_moderate, PS3_supporting, PM2_supporting, PM3_very-strong (PAH VCEP specifications version 2.0.0; approved July 16, 2024).