The zebrafish no tail gene (ntl) is indispensable for the formation of the notochord and tail structure. Here, we show the presence of an intronless ntl gene in zebrafish, which we designated cryptail (ctl). ctl could not be found in any zebrafish genomic resources examined and was only just cloned by a PCR-based approach that relied on its lack of introns and homology to ntl. The amplifiable region of ctl was confined to the transcribed region of ntl. ctl thus appeared to have been generated by reverse transcription of ntl mRNA, like a processed pseudogene. ctl was very polymorphic even in individual fish, but had no missense mutations. This may suggest that ctl has a physiological function in zebrafish. (C) 2003 Elsevier Science (USA). All rights reserved.