Mutagenic chain reaction (MCR) mechanism driving the first mosquito CRISPR–Cas9 gene drive (CGD) (Gantz et al. 2015). (Left) The drive construct contains sequences for gRNA (red), Cas9 (cyan), and Plasmodium falciparum–targeting single-chain antibodies (orange). Cas9 scans for the PAM sequence, and then gRNA checks for a complementary sequence. When the target sequence is found, the Cas9–gRNA complex cleaves (black and white arrow) the wild-type allele (dark blue). (Middle) The double-stranded break is repaired by homology-directed repair (HDR). (Right) Following HDR, the mosquito is homozygous for the drive construct.