MCL1 and small cell lung carcinoma: Next, we focused on targeting BCL-XL and MCL-1 co-dependent cells, as these types of SCLC tumors are highly resistant to chemotherapy and cannot be safely eradicated with the currently available inhibitors of BCL-2 family proteins because the combination of BCL-XL and MCL-1 inhibitors causes severe normal tissue toxicities and lethality in mice [23, 24, 26].