TBC1D20 and Micro syndrome: Furthermore, Warburg Micro syndrome‐4 (WARBM4) (OMIM#615663) caused by homozygous mutation of the TBC1D20 gene has been associated with corpus callosum hypoplasia and severe mental retardation (Liegel et al., 2013; Martsolf, Hunter, & Haworth, 1978; Warburg, Sjo, Fledelius, & Pedersen, 1993).