MCL1 and B-cell chronic lymphocytic leukemia: Using RT-PCR with specific primers for exon 1 and exon 3 of MCL1, we showed that sudemycin D1 upregulated MCL1-SHORT splice variant, which lacks exon 2, while decreasing MCL1-LONG, already at 30 minutes of treatment, both in SF3B1-unmutated and -mutated CLL cases (Figure 3D).