a, A classical operon, found in prokaryotes (such as bacteria). Several genes, often functionally related, form a tight cluster on the genome. Operons are under the control of regulatory elements (promoter, operator) and factors that bind to these elements. Transcription of the cluster results in a single molecule, a multi-gene transcript of messenger RNA, which codes for several proteins and is directly translated into distinct protein products. b, An operon in the nematode Caenorhabditis elegans, typical of the several hundred discovered by Blumenthal et al.3 in the first genome-wide survey for operons in a eukaryotic organism. The initial multi-gene transcript is split up into separate messenger RNAs by trans-splicing, concomitant with the insertion of splice leader sequences (SL1, SL2). Although these operons fit the classical genetic definition4, the processing of their transcripts into proteins differs fundamentally from that of prokaryotic operons.