Three types of alternative transcription starts identified in this study. (a) ME-Exon: mutually exclusive starting exons (Sgk; GenBank:AK132234 and GenBank:AK086892). (b) Intronic: starts within introns that run into the next exon (Egfr; GenBank:AF275367 [longer form] and GenBank:AK087861 [shorter intronic start form]). (c) Exonic: starts within exon of longer transcript (Ntrk1; GenBank:AK081588 and GenBank:AK148691; supported by a CpG island and 5'-RACE). 5'-RACE, 5' rapid amplification of cDNA ends.