Abundance of miR-133a 5′ isomiRs in atrial tissue. MiR-133a-3p (green) and miR-133a-5p (blue) isomiRs were evaluated by deep sequencing of human (A) and murine (B) atrial tissues. 5′ isomiRs affect the seed sequences and are considered functionally more important than 3′ isomiRs. The most abundant 5′ isomiRs, with the most frequently-occurring 3′variant for each of these, are shown. MirBase-annotated miR-133a-3p and 5p sequences are shown in black with ‘seed’ regions (nt 2–8) underlined. Note that there is no miRBase annotation for human miR-133a-5p. (C) Venn diagram showing numbers of human target mRNAs predicted by TargetScan with the seed regions of the 2 most abundant human miR-133a-3p isomiRs and the single abundant miR-133a-5p isomiR used as inputs. (D) (Left panel) Murine target mRNAs predicted by TargetScan with seed regions of the 2 abundant murine miR-133a-3p isomiRs and single abundant miR-133a-5p isomiRs as inputs. (Right panel) Venn diagram showing numbers of putative miR-133a-3p and miR-133a-5p target mRNAs determined by cross-referencing predicted murine targets with genes demonstrated to be differentially regulated in the hearts of miR-133a knockout mice .