Our results strongly support a role for the DRD2 and PPP1R1B genes in susceptibility to autism spectrum behaviours in males from affected sib-pair families in which there are only affected males, and especially those males where there are severe impairments in social interaction (DRD2 and PPP1R1B), verbal communication (DRD2), nonverbal communication (PPP1R1B) and stereotyped behaviours (DRD2 and PPP1R1B). Here, DRD2 is linked to autism.