When the 50% growth-inhibitory concentrations (IC50) of the two were compared with a leukemia cell line with a 20-fold resistance to cytarabine (HL/ara-C20), it was discovered that clofarabine had only a six-fold resistance, and protein expression analyses found that hCNT1 and deoxycytidine kinase (dCK) expression levels were reduced in HL/ara-C20 when compared to normal human myeloid leukemia cells (HL-60). This evidence concerns the gene DCK and leukemia.