Designed peptides and small molecules have achieved high affinity and excellent specificity for Bcl-2 (Souers et al., 2013), Bcl-xL (Leverson et al., 2015a), Mcl-1 (Lee et al., 2008; Foight et al., 2014; Leverson et al., 2015b), and Bfl-1 (Dutta et al., 2013), and highly specific small molecule inhibitors of Bcl-2 and Bcl-xL (ABT-199 and A-1155463) have defined the dependency of ABT-263-sensitive cancer cell lines on Bcl-2, Bcl-xL or both (Leverson et al., 2015a). This evidence concerns the gene MCL1 and cancer.