The ADAR enzymes mediate the hydrolytic deamination of adenosines in specific RNA substrates and thereby diversify both the transcriptome and the proteome in metazoan species. Three promoters drive the transcription from the ADAR1 gene yielding the ADAR1-A, -B, and -C transcripts, which, in turn, lead to the production of two protein isoforms, namely, iADAR1 and cADAR1. In this study, we establish the presence of a previously unidentified alternative intron within the 5'-end of the common second exon of mRNAs encoding ADAR1 in primate species-a region that can function either as a 5'-UTR or an ORF. In addition, it is shown that the relative expression of the three promoter-specific ADAR1 transcripts is tissue specific and that the novel intron is excised from all transcripts, but at different relative levels indicating a specific regulation of the alternative splicing. Finally, possible functional consequences of the splicing are investigated. From these studies, we conclude that the alternatively spliced ADAR1-A transcript is immune to nonsense-mediated decay although it is a potential substrate. Moreover, this transcript is associated with translating ribosomes, which suggests that a truncated version of iADAR1 is expressed.