We found a significant inhibition of drug-induced apoptosis in presence of ET-1 both in CLL cultured alone and in CLL in co-culture on endothelial layer, with an increase in viability from 25%±7% and 28%±6% in fludarabine-treated CLL to 31%±8% and 37%±6% in CLL treated with fludarabine and ET-1, respectively (p = 0.012 and p = 0.003, Figure 3). The gene discussed is EDN1; the disease is B-cell chronic lymphocytic leukemia.