(A) Schematic diagram of the 5′ region of the PML gene showing the two major breakpoint regions (designated by vertical arrows) involved in formation of the chimeric PML-RARα fusion gene. PML exons (rectangles) are numbered. (B) Schematic drawing of the PML-RARα ‘L’ and ‘S’ isoforms, created by fusion of the RARα B through F domains with PML exon 6 or PML exon 3, respectively. Motifs in the PML gene common to each isoform include a Ring finger (Ring) followed by two B-boxes (B1 and B2) and a coiled-coil domain consisting of four distinct coils. A putative nuclear localization signal (NLS) and a serine/proline-rich region (S/P) are located in PML exon 6 and are not present in the PML-RARα S isoform.