Meis1b and Pbx1b cannot induce apoptosis in caspase 8–null Jurkat T cells. Wild-type and caspase 8–null Jurkat T cells were transiently transfected with murine Meis1b (A) or murine Pbx1b (B) and were assayed for cells undergoing apoptosis at 1-day intervals after transfection. (A-B) Transfection efficiency, 38%. (C) Nuclear lysates from stably transfected Jurkat caspase 8–null cells were separated on a 10% SDS page gel, blotted onto PVDF membrane, and probed with Meis1 antibody 78-5. Nontransfected Jurkat caspase 8–null nuclear extracts were included to assess baseline expression of Meis1 in these cells. Cytoplasmic extract was included for the cell line transfected with Meis1bΔM2.