The present study has investigated the regulation of malignant phenotype and gene expression in a human promyelocytic leukemia cell mutant (HL-60-AR) by means of cellular engineering technique of cybridization between the fusions of the mutant cells with enucleated mouse reticulocytes. Results indicate that the cybrid cells (HL-R) incorporated with reticulocyte cytoplasts become differentiated, and the malignancy is obviously suppressed or reversed to a certain degree when compared with those of parental tumor cells. They lose the growth ability to form colony soft agar medium, become non-tumorigenic under heterotransplantation to nude mice, and are accompanied by decrease in growth rate, cellular mitotic index and DNA synthesis. No gene transcripts or homologous sequence of mRNA corresponding to c-myc oncogene can be detected in cybrid cells by Northern blot technique with cloned cmyc gene probe, suggesting that the expression of originally active c-myc has been inhibited. On the other hand, analysis, The present study has investigated the malignancy phenotype and gene expression in a human promyelocytic leukemia cell mutant (HL-60-AR) by means of cellular engineering technique of cybridization the fusions of the mutant cells with enucleated mouse reticulocytes. Results indicate that the cybrid cells (HL-R) incorporated with reticulocyte cytoplasts became differentiated, and the malignancy is obviously suppressed or reversed to some certain when those with parental tumor cells. They lose the growth ability to form colony soft agar medium, become non -tumorigenic under heterotransplantation to nude mice, and are accompanied by decrease in growth rate, cellular mitotic index and DNA synthesis. No gene transcripts or homologous sequence of mRNA corresponding to c-myc oncogene can be detected in cybrid cells by Northern blot technique with cloned cmyc gene probe, suggesting that the expression of originally active c-myc has been inhibited. On the othe r hand, analysis,