Western blot analysis of combined exogenous and endogenous p21 protein and exogenous p21 protein expression from A549, A549/p21 clone 1, 29, 30, and 32 cell lysates. A) Total p21 protein was analyzed in parental A549 or A549/p21 clones, which express stable low-level exogenous p21. Twenty μg of lysate were blotted on a PVDF membrane and incubated with anti-Waf1 (p21) (Ab-1) primary monoclonal antibody. B) Exogenous p21 protein was confirmed in the A549/p21 clones. Twenty μg of lysate were blotted on a PVDF membrane and incubated with anti-His Tag monoclonal antibody recognizing the Hexa-Histidine fusion tag of p21 transactivated from the expression vector contained in the stable A549/p21 clones. The asterisk indicates the non-specific antigen/antibody interaction.