MCL1 and cancer: The human apoptotic regulatory pair BID/MCL‐1 is a target for cancer drug discovery.22 The BH3 binding domain of BID (BID80–102) adopts a helix on binding to a surface groove on MCL‐1 (Figure 3 a).15b, 16 Five single Cys variants of BID80–102 (Figure 3 a) were synthesised (see Supporting Information), each with a unique Cys residue in a different position.