Cytotoxicity was assessed by MTT, and modulated by simultaneous addition of 2'-deoxycytidine (dCK natural substrate), tetrahydrouridine (CDA competitive inhibitor) and diethylpyrocarbonate (cN-II non-competitive inhibitor), while the expression of hENT1, dCK, cN-II, CDA and RR in WIL2-S, Jurkat and CCRF-CEM cells as well as in lymphoid cells from 25 chronic lymphocytic B-leukemia (B-CLL) patients was studied with quantitative-PCR.