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In the February 22 Nature Labrador et al. challenge the central dogma that only one of the DNA strands is transcribed into a single precursor RNA and then translated into protein (Nature 2001, 409:1000). They analysed transcripts at the modifier of mdg4 mod(mdg4) locus of Drosophila using RNase protection. It appears that the major 2.2 kb transcript is generated by the fusion of two precursor RNAs: exons I to IV are transcribed from one DNA strand, while exons V and VI are transcribed using the complimentary strand as the DNA template. Labrador et al. propose that the two transcription units running in opposite orientations create two partially complimentary RNA precursors that may be joined by a trans-splicing mechanism. Such a scenario has implications for our understanding of eukaryotic genome complexity and evolution.

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