The back-fusion junctions (BFJs) of circRNAs from genic regions may have three possible positions relative to the nearest exon-intron boundaries of their cognate linear transcripts, giving rise to three circRNA subtypes:

(1) canonical (or completely boundary) circRNAs with both BFJs aligning to exon-intron boundaries;

(2) half interior (or half boundary) circRNAs with one, but not both, of the BFJs aligning to exon-intron boundaries;

(3) complete interior (or non-boundary) circRNAs with both junctions appearing inside of exons or introns.

Lariats are byproducts of splicing and can be regarded as variants of canonical circRNAs; for clarity, we list lariats separately here.

Intergenic circRNAs, derived from intergenic regions that often encode poorly annotated noncoding transcripts, can be considered a new distinct subtype of their own.

We collectively refer to complete interior, half interior, and intergenic circRNAs as noncanonical circRNAs (nc-circRNAs) to distinguish them from canonical circRNAs (c-circRNAs).