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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 $5^{\prime }$ splice site and the acceptor a $3^{\prime }$ splice site; those labels describe splice-boundary roles, not the order of their coordinates along the precursor.

For a precursor with ordered exons $1\to 2\to 3\to 4$, a circle containing exons 2 and 3 has the ordinary internal junction $2\to 3$ and a circularizing junction $3\to 2$. A sequence read spanning $3\mid 2$ 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.^[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.

A circular RNA has a covalently closed RNA backbone with no free $5^{\prime }$ or $3^{\prime }$ end. The closure concerns the chain of phosphodiester bonds, not whether a drawing happens to look round. A linear RNA folded into a stem-loop still has two ends. Circular RNAs can arise through RNA ligation; some cellular examples are generated by back-splicing of transcribed sequences.^[ref][ref]

One internal cut in the circle produces a single linear chain. One internal cut in an already linear chain produces two fragments, assuming no other links hold the pieces together. This connectivity difference explains why enzymes requiring a free end can distinguish the molecules, although circularity does not make an RNA immune to every cleavage process.

A 2013 study combined RNA sequencing with and without treatment by RNase R, an enzyme acting from RNA ends. Enrichment of back-splicing junction reads, together with additional validation, supported a substantial population of endogenous circular molecules.^[ref] Combining chemical resistance with junction evidence is more informative than classifying a molecule solely by its apparent shape or persistence after one treatment.

William Richard Jeck is a researcher credited with identifying and validating endogenous circular RNA species. His Duke institutional profile gives the full name, and the corresponding institutional publication list directly links him to the 2013 circular-RNA paper published under William R. Jeck.^[ref]

Jeck and collaborators compared RNA sequencing results from samples treated with RNase R and corresponding untreated samples. The approach enriched for molecules resistant to degradation of linear RNA and detected junctions inconsistent with ordinary linear exon order. Complementary assays tested the circular interpretation. The paper also reported an association with complementary Alu elements in bordering introns; that association alone is not a complete reaction mechanism for every circle.^[ref]

A numerical example clarifies what enrichment means. Imagine an original mixture of $10$ circular and $990$ linear RNA molecules, so circles comprise $1\%$. After a hypothetical treatment, suppose $8$ circular and $2$ linear molecules remain. Circles now comprise $8/(8+2)=80\%$ of the recovered mixture, an $80$-fold increase in their fraction, even though their absolute count decreased by $20\%$. These invented counts assume that molecules are counted comparably and are not measurements from the paper. They show why a large proportional enrichment must not be interpreted as treatment having created more circles.

The same care applies to a candidate back-splicing junction: identifying an unusual sequence order and proving that its source molecule is circular are separate steps. Enzyme resistance, junction evidence and independent product characterization address different alternatives. Jeck's credited contribution is therefore more informative than a catalogue of surprising sequencing reads: it illustrates how computational discovery and biochemical validation can constrain one another without making every resistant transcript a proven circular RNA.