| Common insert-size window | Approximately 18–30 nucleotides (nt) | This window includes mature miRNAs and many other short RNA species while limiting the amount of longer RNA carried into the library. | Treat this as a protocol choice, not a universal boundary; the optimal range depends on the RNA classes being studied. |
| MicroRNAs (miRNAs) | Mature miRNAs are commonly about 20–24 nt; many are around 21–23 nt. | A size-selection window beginning near 18 nt generally captures most mature miRNAs. | Very narrow size selection can exclude biologically relevant short or longer variants and may reduce recovery. |
| PIWI-interacting RNAs (piRNAs) | Often about 24–31 nt, with length distributions varying by organism and tissue. | An 18–30 nt window captures many piRNAs but can exclude molecules at or above its upper cutoff. | For piRNA-focused work, consider extending the upper size boundary and validating it against the study organism. |
| Transfer-RNA-derived fragments | Sizes vary substantially; some fragments fall within 18–30 nt, while others are shorter or longer. | These fragments can be underrepresented if the size-selection window is too restrictive. | Choose the range according to the fragment types of interest and consider RNA modifications that may affect reverse transcription. |
| Other small ncRNA fragments | Fragments from sources such as Y RNAs and small nucleolar RNAs can span a broad range of lengths. | Short-RNA libraries can capture fragments as well as intact small RNA classes. | Interpret detected fragments in light of the size cutoff and the library’s ligation and reverse-transcription chemistry. |
| Insert length versus library-fragment length | The insert is the RNA-derived sequence; the sequenced library fragment also includes adapter-derived sequence. | Confusing insert size with total fragment size can lead to incorrect gel-band selection. | Calculate the expected adapter-plus-insert size for the specific protocol before selecting a gel region. |
| Size-selection strategy | Gel-based or other validated size-selection methods may be used, depending on the workflow. | Selection defines which RNA-derived lengths are retained and can affect library yield and composition. | Use consistent selection boundaries across samples; check whether adapter dimers are separated from the desired library fragments. |
| Adapter-ligation bias | Observed representation can depend on RNA end structure, terminal nucleotides, and adapter-ligation conditions. | Sequencing counts may not directly reflect the original abundance of every RNA species. | For quantitative comparisons, use the same preparation conditions across samples and consider suitable synthetic controls. |
| Quality-control checks | Assess library yield, fragment-size distribution, and the presence of adapter dimers before sequencing. | These checks help identify poor recovery, unwanted short products, or inconsistent size selection. | Interpret the measured fragment profile with the protocol’s adapter lengths in mind. |