pulse before each cuticular molt in Drosophila. In vertebrates The let-7 family has a lot more members in vertebrates than in C.elegans and Drosophila. The sequences, expression timing, as well as genomic clustering of these miRNAs members are all conserved across species. The direct role of let-7 family in vertebrate development has not been clearly shown as in less complex organisms, yet the expression pattern of let-7 family is indeed temporal during developmental processes. Given that the expression levels of let-7 members are significantly low in human cancers and cancer stem cells, the major function of let-7 genes may be to promote terminal differentiation in development and tumor suppression. Regulation of expression Although the levels of mature let-7 members are undetectable in undifferentiated cells, the primary transcripts and the hairpin precursors of let-7 are present in these cells. It indicates that the mature let-7 miRNAs may be regulated in a post-transcriptional manner. By pluripotency promoting factor LIN28 As one of the genes involved in (but not essential for) induced pluripotent stem (iPS) cell reprogramming, LIN28 expression is reciprocal to that of mature let-7. LIN28 selectively binds the primary and precursor forms of let-7, and inhibits the processing of pri-let-7 to form the hairpin precursor. This binding is facilitated by the conserved loop sequence of primary let-7 family members and RNA-binding domains of LIN28 proteins. Lin-28 uses two zinc knuckle domains to recognize the NGNNG motif in the let-7 precursors, while the Cold-shock domain, connected by a flexible linker, binds to a closed loop in the precursors. On the other hand, let-7 miRNAs in mammals have been shown to regulate LIN28, which implies that let-7 might enhance its own level by repressing LIN28, its negative regulator. In autoregulatory loop with MYC Expression of let-7 members is controlled by MYC binding to their promoters. The levels of let-7 have been reported to decrease in models of MYC-mediated tumorigenesis, and to increase when MYC is inhibited by chemicals. In a twist, there are let-7-binding sites in MYC 3' untranslated region(UTR) according to bioinformatic analysis, and let-7 overexpression in cell culture decreased MYC mRNA levels. Therefore, there is a double-negative feedback loop between MYC and let-7. Furthermore, let-7 could lead to IMP1(/insulin-like growth factor II mRNA-binding protein) depletion, which destabilizes MYC mRNA, thus forming an indirect regulatory pathway. Targets of let-7 Oncogenes: RAS, HMGA2 Let-7 has been demonstrated to be a direct regulator of