The c.557C>T variant in PAH is a missense variant predicted to cause substitution of threonine by isoleucine at amino acid 186 (p.Thr186Ile). At least one patient with this variant displayed plasma phenylalanine concentration persistently above 120umol/L (2mg/dL), which is highly specific for PAH deficiency; PAH VCEP Expert consensus is that BH4 deficiency was sufficiently excluded (PP4_Moderate, PMID: 30459323). This individual was compound heterozygous for the variant and a pathogenic variant (0.5 PM3 points, PM3_supporting). The allele frequency in gnomAD v4.1.0 is 0.000001239, which is lower than the ClinGen PAH Variant Curation Expert Panel threshold (<0.0002) for PM2_Supporting, meeting this criterion (PM2_Supporting). The computational predictor REVEL gives a score of 0.87, which is above the threshold of 0.773 but below 0.932, evidence that correlates with moderate impact to PAH function (PP3_Moderate). In summary, this variant meets the criteria to be classified as Like;y pathogenic for autosomal recessive PAH deficiency based on the ACMG/AMP criteria applied, as specified by the ClinGen PAH Variant Curation Expert Panel: PP3_moderate, PP4_moderate, PM2_supporting, PM3_supporting (PAH VCEP specifications version 2.0.0; approved July 16, 2024).