(A) The genomic organization and primary transcript structures of the human miR-17-92, miR-106a-363, and miR-106b-25 clusters. The miR-106a-363 primary transcript has not been characterized. (B) Based on their seed sequences, the region considered most important for target selection (nucleotides 2–7; shown in blue), the miRNAs of these clusters can be grouped into four families: the miR-17 family (miR-17, miR-20a/b, miR-106a/b, and miR-93); the miR-18 family (miR-18a/b); the miR-19 family (miR-19a/b); and the miR-25 family (miR-25, miR-92a, and miR-363). MicroRNAs of the miR-17-92 cluster and its paralogs have been implicated in normal development of the heart, lungs and immune system as well as in tumorigenesis.