Although we were readily able to establish RB1 knockout lines, all BRCA1 targeted clones were hemizygous for BRCA1 deletion and retained BRCA1 expression (Supplementary Table S19), suggesting that engineered homozygous loss of BRCA1 was cell lethal, even in a tumor type in which BRCA1 loss is frequently observed (68). This evidence concerns the gene BRCA1 and neoplasm.