Gene structure of AF4, containing the exon/intron structure as well as the distances of all three first exons (1a is encoded by two exons; 1b and 1c) and their distances from each other (unpublished data). There is also a stop in intron 3 (as designated) and an alternative splice of exon 18 to the 3-NTR, skipping exon 19 and 20. Therefore this protein comes in different flavors, as there are presumably three independent promotor, and one carboxy-terminal exon skipping.