For in vivo studies, we used CRISPR-Cas9 targeting in mouse embryos to generate a humanized PKU model, in the C57BL/6J background, in which we replaced a small portion of the endogenous mouse Pah exon 7 with the orthologous human sequence spanning the PAH1 and PAH2 protospacer/PAM sequences and containing the P281L variant (Fig. 2a). Here, PAH is linked to phenylketonuria.