To gain more confidence of using the RS-1 supplementation to improve nuclease-mediated knock-in efficiency in the embryos, we tested Cas9-mediated knock-in of CFTRdelF508, the most frequent mutation type identified in human cystic fibrosis patients, to the rabbit CFTR locus (Supplementary Fig. 2). The gene discussed is CFTR; the disease is cystic fibrosis.