a | The principal structures of a long non-coding RNA (lncRNA) to be annotated. Annotations are hierarchical: they are composed of gene loci, each of which is composed of one or more partially overlapping transcripts, themselves composed of one or more exons (blue rectangles). b | Positional classification of lncRNAs with respect to the nearest protein-coding gene. Genic lncRNAs overlap a protein-coding gene locus, whereas intergenic lncRNAs, also known as long intergenic non-coding RNAs (lincRNAs), do not. Transcripts that overlap a protein-coding gene on the opposite strand are identified as antisense. PAS, polyadenylation site; TSS, transcription start site.