PTBP1 and chronic myelogenous leukemia, BCR-ABL1 positive: Indeed, when we used siRNAs to downregulate PTBP1, we observed diminished induction of imatinib-induced apoptosis in CML cells, as well as impaired induction of BIM isoforms that harbor the BH3 domain encoded in exon 4 (BIML and BIMS) (Fig. 9A).