The t(15;17)(q22;q12) translocation is the cytogenetic hallmark of acute promyelocytic leukemia (APL). The PML and retinoic acid receptor-alpha (RAR alpha) transcription factor genes are involved at translocation breakpoint. To elucidate the biologic function of PML, antipeptide antibody against PML protein was raised in rabbits. This antibody was able to detect a 90-kD PML protein and a 110-kD PML-RAR alpha fusion protein by Western blotting and a nuclear speckled pattern in all cell lines by immunofluorescent staining. In K562 and NIH/3T3 cells transfected with a PML expression plasmid, we found PML to be associated with the nuclear matrix. Our results also showed that PML is a phosphorprotein. A weak signal was detected in a Western blot containing the immunoprecipitated PML protein using the phosphotyrosine-specific monoclonal antibody. Therefore, at least one of the sites was phosphorylated by a tyrosine kinase. From our analysis of the phosphoamino acids of the PML protein by complete hydrolysis and thin-layer chromatography, we concluded that both tyrosine and serine residues of PML are phosphorylated. To investigate whether expression of the PML protein is cell-cycle related, HeLa cells synchronized at various phases of the cell cycle were analyzed by immunofluorescence staining and confocal microscopy for PML expression. We found that PML was expressed at a lower level in S, G2, and M phases and at a significantly higher level in G1 phase. Our study showed that PML has many similar properties compared with the tumor suppressor, eg, Rb. These findings further support the important role of PML in APL pathogenesis.