ARID1A and microcephaly: Conversely, haploinsufficiency of ARID1B (also known as BAF250B), a mutually exclusive paralog of ARID1A, similarly results in microcephaly [119,120,121], implying antagonistic roles in NSPC proliferation—with ARID1A-BAF functioning as a repressor and ARID1B-BAF as an activator.