Plasmids expressing foldon–GFP–PCP [a fusion protein consisting of the PP7 coat protein (PCP), green fluorescent protein (GFP) and a T4 fibritin trimeric motif foldon] were gifts from Chun-Qing Song (14 (link)). The single guide RNAs (sgRNAs) used in this study were constructed by modifying pX330-mitf (Addgene, 69801) or pLH-sgRNA1-2XPP7 (Addgene, 75390). The fragments of sgRNA were subcloned into the backbone vector using the NEB Gibson assembly kit (NEB, E2611L). All gRNAs in this research are listed in Supplementary Tables S1 and S2.