ER stress molecule ASK1 is necessary for flavopiridol-induced cell death. Viability of CLL cells (n = 5) untransfected or transfected with scrambled or ASK1 siRNA, then treated with flavopiridol (2μM), F-ara-A (5μM), thapsigargin (1μM), or tunicamycin (3 μg/mL) 24 hour after transfection. Flavopiridol cytotoxicity decreased in the presence of ASK1 siRNA (P = .0005).