It has been proposed that isoforms of Drosophila mod(mdg4) result from the trans-splicing of independent mRNA transcripts derived from both anti-parallel DNA strands. In the August 14 Proceedings of the National Academy of Sciences, Rainer Dorn and researchers at the Martin Luther University, Halle, Germany, provide proof for trans-splicing of mod(mdg4) transcripts in transgenic flies (Proc Natl Acad Sci USA 2001, 98:9724-9729). They characterized 26 different classes of mod(mdg4) transcripts all containing a common 5' sequence (exons 1-4). Analysis of mod(mdg4) isoforms and the entire 28 kb mod(mdg4) locus provided further evidence for trans-splicing of mRNA from both strands. To prove the splicing phenomenon formally, Dorn et al. generated transgenic flies expressing tagged mod(mdg4) exons from different chromosomal loci. The presence of spliced transcripts was confirmed using RT-PCR analysis. They also found evidence for independent, isoform-specific promoters. This study provides an interesting animal system to investigate the mechanisms of trans-splicing in vivo.