Cas9 mRNA and single guide RNAs (sgRNAs) were synthesized as described previously [5 (link)34 (link)]. Briefly, Cas9 mRNA was prepared in vitro from linear DNA templates, using the mMESSAGE mMACHINE T7 Ultra Transcription Kit (Invitrogen, Carlsbad, CA, USA), and sgRNAs were generated using the MEGAshortscript T7 Transcription Kit (Invitrogen), both according to the manufacturer's instructions. The following sequences were used for sgRNA synthesis: left sgRNA, TTGAGACCTCGCATCGAAGATGG; right sgRNA, TGTGATTGCTTCAGTGACTGCGG.