To induce a DSB close to the most frequent mutation in CEP disease (c.217 T > C in UROS exon 4, chromosome 10), we designed a sgRNA inducing a DSB near the c.217 position and devised a 181nt-ssODN carrying a silent SacI restriction site close to the c.217 T position (Fig. 1b). This evidence concerns the gene UROS and Congenital erythropoietic porphyria.