Overview of the wild-type and Y223F mouse Btk locus after CRISPR-Cas9–mediated gene editing. The Y223F substitution was introduced into exon 8 of the Btk gene using CRISPR-Cas9–mediated gene editing. Additional silent mutations were introduced to create an Afel restriction site (AGCGCT) for analytical purposes and to avoid further editing of the locus. The sequence of the single guide RNA (sgRNA, green), silent point mutations (red), position of amino acid 223 (grey), and the introduced Afel site (yellow) are indicated.