A single mammalian transcript normally encodes one protein, but the transcript of GNAS (G-protein alpha-subunit) contains two reading frames and produces two structurally unrelated proteins, XLalphas and ALEX. No other confirmed GNAS-like dual-coding transcripts have been reported to date, even though many such candidate genes have been predicted by bioinformatics analysis. In this study, we constructed a series of vectors to test how two protein products were translated from a single transcript in vitro. The length of the ORF (open reading frame), position of the first AUG and the Kozak motif were found to be important factors. These factors, as well as 55-bp NMD (nonsense-mediated mRNA decay) rule, were used in a bioinformatics search for candidate dual-coding transcripts. A total of 1307, 750 and 474 two-ORF-containing transcripts were found in human, mouse and rat, respectively, of which 170, 89 and 70, respectively, were found to be potential dual-coding transcripts. Most transcripts showed low conservation among species. Interestingly, dual-coding transcripts were significantly enriched for transcripts from the zinc-finger protein family, which are usually DNA-binding proteins involved in regulation of the transcription process.