The tumor suppressor promyelocytic leukemia (PML) was first identified as a fusion partner of human retinoic acid receptor alpha (RARα) as the result of a chromosomal translocation discovered in acute PML (APL) (de The et al., 1991; Kakizuka et al., 1991). The gene discussed is RARA; the disease is progressive multifocal leukoencephalopathy.