Small open reading frames (sORFs) and RNA. Box: Within mRNA that encodes canonical protein coding sequences (CDSs), sORFs can appear in the 5′ UTR (upstreamORF [uORF]), initiating in the 5′ UTR and extending into the CDS in an alternative reading frame (upstream overlapping ORF [u.oORF]), in the 3′ UTR (downstreamORF [dORF]), or nested within the CDS in an alternative reading frame. sORFs can also be found in long noncoding RNA (lncRNA, bottom) and circular RNA (circRNA, right), as well as additional classes of RNA not pictured.