Time-dependent inhibition of 1 μmol/L RA-induced PML/RARα degradation by 100 μmol/L DEVD in (A) NB4 cells and (B) Zn-induced U937/PR9 cells. Immunoblot analysis was performed on total cellular proteins using an anti-RARα antibody (Santa Cruz Biotechnology). β-tubulin antibody was used for loading control. (C and D) Effect of DEVD and zVAD on cell differentiation in cells grown in the absence (black bars) or in the presence of 1 μmol/L RA for 96 hours (diagonal bars) evaluated by NBT reduction test (C) or quantitative expression of CD11a antigen (D).