The promyelocytic leukemia (PML) gene is a gene known to be a tumor suppressor, although recent data suggest that it has a dual function in tumorigenesis. It was initially discovered in acute promyelocytic leukemia (APL) in which a t(15; 17) chromosomal translocation fused it to the retinoic acid receptor alpha ($RAR{alpha}$). It has been shown to be involved in various types of cancer. It has at least 6 nuclear isoforms and a cytoplasmic type with different characteristics. Its multiple functions in growth inhibition, apoptosis induction, replicative senescence, inhibition of oncogenic transformation, and suppression of migration and angiogenesis have made it a therapeutic target for cancer therapy. However, its dual role in the process of tumorigenesis has made this field challenging. In this review, we discuss PML structure, functions and expression in tumors.