Identifying (non-)coding RNAs and small peptides: challenges and opportunities.

Over the past decade, high-throughput studies have identified many novel transcripts. While their existence is undisputed, their coding potential and functionality have remained controversial. Recent computational approaches guided by ribosome profiling have indicated that translation is far more pervasive than anticipated and takes place on many transcripts previously assumed to be non-coding. Some of these newly discovered translated transcripts encode short, functional proteins that had been missed in prior screens. Other transcripts are translated, but it might be the process of translation rather than the resulting peptides that serves a function. Here, we review annotation studies in zebrafish to discuss the challenges of placing RNAs onto the continuum that ranges from functional protein-encoding mRNAs to potentially non-functional peptide-producing RNAs to non-coding RNAs. As highlighted by the discovery of the novel signaling peptide Apela/ELABELA/Toddler, accurate annotations can give rise to exciting opportunities to identify the functions of previously uncharacterized transcripts.