IRF1 and leukemia: Moreover, it has been seen that RA may be involved in extrinsic apoptotic signaling in postmaturation NB4 APL cells through the induction of expression of interferon regulatory factor-1 (IRF-1), which in turn upregulates the death ligand TRAIL [100,101]; in human lung cancer and in leukemia cells, RA can stimulate the expression of the receptor for the death ligand TNFα [102,103].