S-63845 is a potent and selective MCL1 inhibitor (Ki (MCL1, FP) 10.000 1.2 nM; Ki (BCL-XL, FP) > 10.000 1.2 nM). S63845 specifically binds with high affinity to the BH3-binding groove of MCL1. S-63845 potently kills MCL1-dependent cancer cells, including multiple myeloma, leukaemia and lymphoma cells, by activating the BAX/BAK-dependent mitochondrial apoptotic pathway. In vivo, S63845 shows potent anti-tumour activity with an acceptable safety margin as a single agent in several cancers. Moreover, MCL1 inhibition, either alone or in combination with other anti-cancer drugs, proved effective against several solid cancer-derived cell lines. These results point towards MCL1 as a target for the treatment of a wide range of tumours.