Bax and p21 expression play a critical role in ABT-263 and sorafenib combination treatment-induced cell apoptosis. (A) HCT116 Bax-/- cells transfected with pUSE-GFP/pUSE-GFP-Bax for 48 h were incubated with ABT-263 and sorafenib for 24 h. The protein lysates prepared from the cells were then analysed by Western blot, and cell viability was determined. (B) HCT116 p21-/- cells transfected with pHAGE/pHAGE-p21 using the lentivirus-based stable transfection system were incubated with ABT-263 and sorafenib for 24 h. The protein lysates were analysed by Western blot, and cell viability was determined. (C) The U251, A549, Hela and MDA-MB-231 cell lines were treated with 0.4 μM ABT-263 and 6 μM sorafenib either alone or in combination for 48 h, followed by assessment for cell viability using the trypan blue exclusion assay. (D) The expression levels of p21 and Bax in ABT-263 and sorafenib combination treated-sensitive cells (AGS, HCT116WT, Huh7 and BEL7402) compared with the resistant cells (MDA-MB-231, Hela and HCT116 p21-/-) were determined by Western blot. (E) MDA-MB-231 cells transfected with pHAGE/pHAGE-p21 using the lentivirus-based stable transfection system were incubated with 0.4 μM ABT-263 and 6 μM sorafenib for 48 h, and cell viability was then determined. (F) MDA-MB-231 cell protein lysates were analysed by Western blot using antibodies targeting PARP, caspase-9 and GAPDH. **P < 0.01.