Schematic representation of the three major PML/RAR fusion transcripts. Breakpoints on the RAR gene always occur within intron 2. Due to distinct breakpoints on the PML gene (intron 6, exon 6, and intron 3), different segments of this latter are fused to RAR exon 3, resulting in bcr1 (L type), bcr2 (V type), and bcr3 (S type) fusion transcripts, respectively. Numbers in boxes represent PML and RAR exons.