The critical role of collybistin in inhibitory synaptic structure and function was confirmed by the discovery of missense and nonsense mutations, deletions and complex re-arrangements affecting the collybistin gene (ARHGEF9) in patients with X-linked intellectual disability (XLID; Harvey et al., 2004; Marco et al., 2008; Kalscheuer et al., 2009; Lesca et al., 2011; Shimojima et al., 2011; Lemke et al., 2012; de Ligt et al., 2012; Long et al., 2016; Alber et al., 2017; Klein et al., 2017; Wang et al., 2018). This evidence concerns the gene ARHGEF9 and cask-related x-linked intellectual disability.