In both models, a particular allele has an increased chance of being passed on to the next generation because it produces a toxin to kill gametes that do not carry it. (A, B) In the poison-antidote model, cells produce a toxin (shown as skull-and-crossbones) that can be neutralized by an antidote (shown as a pill); the alleles that do not code for either are shown in gray. In the single-gene model (A) the same gene codes for both the poison and the antidote through alternative transcription. Nuckolls et al. show that the gene wtf4 is a selfish driver in Schizosaccharomyces yeasts. Hu et al. show that two other genes in the wtf family (cw9 and cw27) are also selfish drivers. In the two-gene model (B) different genes produce the poison and antidote, as in the fungus Neurospora (Hammond et al., 2012). (C) In the killer-target model, the toxin only destroys cells that contain alleles with a specific target marker (shown here by concentric black circles). This is the case in Drosophila, where the segregation distortion (Sd) allele acts by killing gametes that contain a sensitive Responder (Rsps) marker (Larracuente and Presgraves, 2012).