The deoxycytidine kinase (dck) gene encodes the enzyme responsible for the metabolic activation of the antileukemic drugs cytosine arabinoside (AraC) and 5-aza-2′-deoxycytidine (decitabine, DAC). Thedck locus was analyzed at the chromosomal and the molecular level in a model of rat leukemic cell lines, in which AraC and DAC resistance was induced, that was marked bydck deficiency. At the chromosomal level, karyotype analysis of metaphase spreads revealed the presence of an aberrant 2q+ chromosome in the AraC-resistant cell line and a (Xq;11q) translocation in its subclone RA/7. The DAC-resistant lines were identical to the parental RCL/O. Fluorescence in situ hybridization on normal rat fibroblast metaphase spreads localized the ratdck gene to chromosome 14q21-q22, a region that was not involved in any of the observed karyotypic aberrations. Analysis at the molecular level revealed an identical rearrangement of thedck gene in the AraC-resistant cell line RCL/A and its subclone RA/7 that resulted in the absence ofdck expression, as assessed by RT-PCR. No genomic rearrangements were observed in a DAC-resistant cell line RCL/D or in its subclone RD/1. However, detection of a single-stranded conformation polymorphism (SSCP) allowed the identification of a single C to G substitution (His to Gln) in thedck cDNA of the DAC-resistant RD/1 clone. The data demonstrate that exposure to AraC and DAC induces a resistant phenotype marked by functionaldck deficiency that may be the consequence of mutations occurring in thedck gene.