Biological complexity does not appear to be simply correlated with gene number but rather other mechanisms contribute to the morphological and functional diversity across phyla. Such mechanisms regulate different transcriptional, translational and post-translational processes and include the recently identified transcription induced chimerism (TIC). We have found two novel chimeric transcripts in the chick and quail that result from the fusion of tyrosine hydroxylase (TH) and insulin into a single mature transcript. The th and insulin genes are located in tandem and they are generally transcribed independently. However, it appears that two chimeric transcripts containing exons from both the genes can also be produced in a regulated manner. The TH-INS1 and TH-INS2 chimeras differ in their insulin gene content, and they encode two novel isoforms of the TH protein with markedly reduced functionality when compared with the canonical TH. In addition, the TH-INS1 chimeric mRNA generates a small amount of insulin. We propose that TIC is an additional mechanism that can be employed to further regulate TH and insulin expression according to the specific needs of developing vertebrates.These studies were financed by the grant BFU2004-2352 from the Spanish Ministry of Education and Science (MEC) and the Red de Grupos RGDM G03/212 from the ‘Instituto de Salud Carlos III’ (Spain) to F.P.; and by a grant from the ‘Comunidad de Madrid’ SAL/0647/2004 to C.H.-S. O.B. and A.M. were awarded a fellowship and C.H.-S. was a holder of a ‘Ramón y Cajal’ contract (all from the MEC, Spain). Funding to pay the Open Access publication charges for this article was provided by MEC (Spain).Peer reviewed