The association of common variants in ROBO2 with expressive vocabulary directly implicates the Slit-robo pathway and axon guidance as potential mechanisms for language development, and this mechanism is reflected in the association observed here with IQSEC1. A second example is the gene FOXP2 in which some specific rare genetic variants result in a sub-type of language disorder called childhood apraxia of speech (CAS OMIM #602081) (Lai et al., 2001). Here, FOXP2 is linked to childhood apraxia of speech.