Schematic representation of the intron-exon structure of relevant parts of the RARα and PML genes and of the formation of RARα2 mRNA and of RARα2-PML mRNAs from APL cases with S-form or L/V-forms of PML-RARα mRNA. Introns are represented by heavy lines. The boxes in the top (RARα gene) and lower (PML gene) panels indicate exons. The dashed lines indicate how RARα exons are joined to form RARα1 or RARα2 mRNAs. The asterisk indicates the RARE site in the promoter region of RARα2. Open boxes indicate noncoding regions. The simple hatched areas indicate the coding regions of RARα1 (A1) and RARα2 (A2), which become fused to the variable PML carboxy-termini in RARα-PML mRNAs. Stippled boxes indicate PML exons (P4, P5, P6, P7a, and P7b), which are variably spliced in different PML-containing isoforms. Cross-hatched boxes indicate coding regions that are consistently retained in RARα- or PML-containing transcripts. ckII is the unique casein kinase-II site in constantly retained region of PML exon 7. Arrowheads indicate the site of PCR primers used in the present studies. The perijunctional nucleotide sequence and codon assignment is indicated below the schematic RARα2 and RARα2-PML mRNAs with the sequence derived from RARα exon 3 italicized and the junctional codon (derived from the last nucleotide of RARα2 exon 3 and the first two nucleotides of RARα exon 4/B-region, PML exon 4 or PML exon 7) underlined.