Preferred uridylation of hairpins from an alternative miRNA biogenesis pathway. (A) Pre-miRNA hairpins can be generated by Drosha-mediated cleavage, or by splicing of a short hairpin intron (mirtron). (B) In general, most miRNA reads do not carry untemplated additions. However, many mirtron-3p species exhibit high rates of terminal uridylation, in some cases comprising the dominant species. Shown are three mirtrons used as models in this study. The fraction of uridylated species are calculated with respect to the splicing-derived species. Their corresponding mirtron-5p species essentially lack untemplated modifications, reflecting that uridylation likely occurred at the hairpin stage.