SUMMARYWe examined the PCR differential display analysis of cDNA clones from human leukemia K562 cell line and those of cytosine arabinoside (ara-C) resistant K562 cells. We compared mRNA samples obtained from the two cell lines by amplified restriction fragment length polymorphism (AFLP)-based mRNA fingerprinting; 40 bands were observed in each lane of a gel, and the screened results were five independent, isolated clones. In this study, we isolated the proline synthetase co-transcribed (PROSC) gene from an ara-C sensitive K562 cell line as one of the clones. RNase protection assay and RT-PCR analysis revealed increased expression of PROSC gene in K562 sensitive cells compared with that of ara-C resistant K562 cells. The PROSC gene product is likely a soluble cytoplasmic protein, but its function remains unclear. Thus, the PROSC gene might be related to the sensitivity to ara-C in human leukemia and downregulated in resistant cells.