The miRNA expression profiles in mammalian organs. (a) Distribution of all expressed miRNAs in mouse organs (center), miRNAs expressed only in a particular organ (right), and miRNA enriched in a particular organ at least two-fold over other organs (left). Note that mouse brain-specific miRNAs were detected at higher levels than mouse lung-specific miRNAs. Indeed, four of the seven brain-specific miRNA radioactive signals were detected at least two-fold over background. (b) Northern blots of representative miRNAs with similar expression patterns in organs of mouse and human. (c) Clustering of expressed miRNAs by their similar expression profiles in mouse (Mm) and human (Hs) brain (B), liver (Li), heart (H), skeletal muscle (M), lung (Lu), kidney (K), and spleen (S). Note that some mouse lung-specific miRNAs (miR-19a, miR -130, miR -213) are also expressed in other human organs. Colored bars to the right of miRNA names refer to the corresponding color-coded classes in the right hand pie chart in (a): brain-specific (red), brain-enriched (purple), or lung-specific (light grey).