ABCB1 and cancer: To determine if the inhibition of the growth of the parental and ABCB1- and ABCG2-overexpressing cancer cells by EC16-1/saporin was due to the induction of apoptosis, the parental and ABC-overexpressing cells were incubated with various concentrations of EC16-1/saporin for 24 h and apoptosis was determined as described in the methods.