It is estimated that between 2–3% of the general population have a form of intellectual disability (ID), and the vast majority of those are thought to have genetic causes (Ropers, 2010). Interestingly, mutations in BAF250B, the largest BAF subunit, have emerged as an important factor in non-familial ID. It has been known that rare de novo deletions or translocations involving the BAF250B (ARID1B) gene in chromosomal region 6q are associated with microcephaly, growth delay and frequent agenesis of the corpus callosum (AgCC) (Pirola et al., 1998; Narahara et al., 1991; Sukumar et al., 1999; Hopkin et al., 1997; Oliveira-Duarte et al., 1990; Rivas et al., 1986; Meng et al., 1992; Valtat et al., 1992; Nagamani et al., 2009; Backx et al., 2011). More recently, high-resolution molecular karyotyping and next-generation trio sequencing studies have identified additional chromosomal legions involving the BAF250B gene as well as truncating BAF250B mutations in sporadic ID patients with similar clinical features as above (Hoyer et al., 2012; Halgren et al., 2012); in one of the studies, inactivating BAF250B deletions were found in 8/887 (0.9%) of the
