We have shown that the synthesis of DNA is not only a phenomenon seen in S phase but also in G1 and G2 phases of the cell cycle in K-562 cells after TPA treatment. This DNA synthesis is probably due to megakaryocytic differentiation and consequent increase in the ploidy of these cells. We have demonstrated that once the cells are differentiated, they express large amounts of c-sis transcripts with concurrent shut-off of the c-myc gene. In the case of c-sis, the regulation of the gene is at the transcriptional level, while c-myc is under post-transcriptional regulation.