PAH and phenylketonuria: In this study, we used a modified CRISPR system, which employs the fusion of inactive Cas9 (dCas9) and the FokI endonuclease (FokI-dCas9) to correct the most common variant (allele frequency 21.4%) in the phenylalanine hydroxylase (PAH) gene - c.1222C>T (p.Arg408Trp) - as an approach toward curing phenylketonuria (PKU).