The BAF45 subunit is encoded by a family of four genes (BAF45a, b, c, and d), which have radically different expression patterns. For example, BAF45b is expressed only in postmitotic neurons. In general, these genes are relatively rarely mutated, but BAF45d is amplified in about 17% of breast cancers. As yet, it is not known if this is due to a carrier effect (that is, coamplification of a nearby essential oncogenic gene) or if its amplification could actually play a role in breast cancer progression. This family was discovered during the purification of BAF complexes from the developing CNS (19), and the remarkable neural specificity of BAF45b helped in understanding how combinatorial assembly could generate functionally distinct complexes (19). These proteins contain two PHD fingers of unclear specificity, which might help localize the complex to specific genetic loci, bearing specific histone marks. The BAF60 family (SMARCD1, 2, and 3) is also less frequently mutated in human cancers. SMARCD1 appears to be biallelically mutated in breast cancer, albeit less frequently (87).
