such as the SW13 line. These studies were confirmed by Wong and colleagues (63), who found that several breast cancer cell lines had lost both Brg and Brm at the protein level but seemed to have mutations in only one of the two homologous subunits. The observation that both Brg and Brm could be disrupted and yet cells remain viable and active was unexpected because several biochemical studies using in vitro transcription on nucleosomal templates indicated that these ATPases might be required for RNA polymerase binding, and conflicting reports in yeast indicated that they might or might not be subunits of RNA polymerase II. These early studies were highly limited because sequencing was labor-intensive. Thus, before the dawn of exome sequencing, the common view was that tumor suppression might require loss of function of both alleles of both of the homologous ATPases or loss of both alleles of subunits that were encoded by single genes, such as BAF47.
