MCL1 and chronic myelogenous leukemia, BCR-ABL1 positive: Using low nanomolar concentrations of S63845, a novel highly specific and potent small-molecule MCL1 inhibitor, we could show for the first time that targeting MCL1 in the presence of BCR-ABL1 inhibition exerted a strong antitumorigenic effect on primary human CD34+ CML stem/progenitor cells.