Gel electrophoretic analysis of ethidium bromide-stained DNA products synthesized in primer competition experiments from RARα2 mRNA (upper band) or RARα1 mRNA (lower band). Five picomoles each of upstream primers specific for RARα1 or RARα2 were used with 10 pmol of a common downstream primer anchored in the C-region of RARα to amplify 1 μg-RNA equivalent of cDNA, as described in Materials and Methods. The abbreviations are: M, 100-bp ladder; L1 to L3, products from 3 APL patients with L-form PML-RARα mRNA; V1, product from an APL patient with V-form PML-RARα mRNA; S1 and S2, products from 2 APL patients with S-form PML-RARα mRNA; NB4, product from APL cell line NB4 mRNA; BM, product from normal bone marrow; PB, product from normal peripheral blood. All illustrated APL RNAs were derived from specimens containing greater than 90% blasts plus promyelocytes.