Similar to in sporadic ID, all but one BAF250B mutations in CSS lead to truncation of the polypeptide (Figure 2); it is likely that unknown genetic factors contribute to the exact phenotypic outcome in patients with BAF250B mutations. All mutations in the homologous BAF250A gene are also nonsense or frameshift mutations, and are found in a smaller number of CSS patients who appear to present with greater disease severity than do BAF250B patients. The two homologous subunits, also known as ARID1A/B, contain the DNA-binding AT-rich interaction domains (ARID domains) and can have opposing roles in cell cycle through specific interactions with the E2F family of transcription factors (Nagl et al., 2007). Both homologs are found mutated in a wide variety of human malignancies, with BAF250A being more highly mutated than BAF250B (Kadoch et al., 2013; Ronan et al., 2013). Interestingly, a Drosophila RNAi screen for regulators of the neuroblast fate revealed that knockdown of Brahma, Moira and Osa (homologs of BRM, BAF47 and BAF250, respectively) results in the generation of extra neuroblasts at the expense of differentiated neurons (Neumüller et
