= Back-splicing
<Back-splicing> is an <RNA splicing> arrangement in which a downstream <splice donor site> joins an upstream <splice acceptor site>. In a common circularization event, the retained segment becomes <circular RNA> with a new junction joining a later part of the precursor to an earlier part. The donor is conventionally called a $5'$ <splice site> and the acceptor a $3'$ <splice site>; those labels describe splice-boundary roles, not the order of their coordinates along the precursor.
For a precursor with ordered <exons> $1\to2\to3\to4$, a circle containing exons 2 and 3 has the ordinary internal junction $2\to3$ and a circularizing junction $3\to2$. A sequence read spanning $3\mid2$ thus reports a different adjacency from the reference transcript order. The example specifies a possible connectivity, not a prediction that every such precursor forms this circle.
A 2013 study combined <RNA sequencing> with and without <RNase R> treatment. Reads crossing back-splice junctions were enriched among the more resistant RNA population, and complementary tests supported circular products.<Circular RNAs are abundant, conserved, and associated with ALU repeats by William R. Jeck and others>[Figures1–4, backsplice mapping, treatment comparison and validation]{ref} A non-colinear junction read alone can also raise questions about mapping, another RNA arrangement or genomic structure. Agreement between junction evidence and a test of molecular connectivity makes the circular interpretation stronger; it does not by itself establish the circle's biological function.
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