To understand the mechanism of cellular resistance to the nucleoside analogue cytarabine (1-B-D-arabinofuranosylcytosine, AraC), two resistant derivatives of the human leukemic line CCRF-CEM were obtained by stepwise selection in different concentrations of AraC. CEM/4ÂAraC cells showed low AraC resistance, whereas CEM/20ÂAraC cells showed high resistance. Both cell lines showed similar patterns of crossresistance to multiple cytotoxic nucleoside analogues, with the exception that CEM/20ÂAraC cells remained sensitive to 5-fluorouridine and 2-deoxy-5-fluorouridine. Both cell lines were sensitive to 5-fluorouracil and to a variety of natural product drugs. Although both CEM/4ÂAraC and CEM/ 20ÂAraC cells displayed reduced intracellular accumulation of [ 3 H]AraC, only CEM/4ÂAraC cells showed reduced uptake of [ 3 H]uridine, which was used to assess nucleoside transport activities. Genes encoding proteins known to be involved in nucleoside transport, efflux, and metabolism were analyzed for the presence of mutations in the two cell lines. In CEM/4ÂAraC cells, independent mutations were identified at each allele of human equilibrative nucleoside transporter 1 (hENT1; SLC29A1), one corresponding to a single-nucleotide change in exon 4, the other being a complex intronic mutation disrupting splicing of exon 13. In contrast to CEM/20ÂAraC cells, CEM/4ÂAraC cells did not bind the hENT1/SLC29A1 ligand nitrobenzylmercaptopurine ribonucleoside and lacked detectable hENT1/SLC29A1 protein. In CEM/20ÂAraC cells, independent intronic mutations impairing splicing of exons 2 and 3 were found at each allele of the deoxycytidine kinase gene. These studies point to at least two distinct mechanisms of AraC resistance in leukemic cells. [Cancer Res 2008;68(7):2349-57]