In and results in a similar phenotype (reviewed in (Halder and Johnson, 2011; Pan, 2010)). the most fundamental mechanisms supporting multicellularity, are those that ensure the proper size and shape of tissues and organs to meet the need of functionality. However, despite intensive investigations into the underlying principles behind a preset size of organs, we are far from having a clear picture to this basic question in developmental biology. Nevertheless, investigations of the Hippo pathway on organ size control have shed light into this mystery (Halder and Johnson, 2011; Pan, 2010; Yu and Guan, 2013). In 1995, two studies in discovered that deletion of ((((phenocopy mutants with regards to tissue overgrowth. Hpo, Sav, Wts, and Mats interact genetically and physically, and the remarkable organ size phenotype elicited by their mutation is unprecedented in other established developmental pathways, thus they were grouped into a new signaling module the Hippo pathway named after the enormous size of mutant organs which resemble that of a hippopotamus. (are (also called (((also called suppresses the overgrowth phenotypes of mutants (Huang et al., 2005). In mice, deleting also diminishes the overgrowth phenotypes caused by deficiency of or other upstream regulators (Zhang et al., 2010; Zhou et al., 2011). Thus, Yki and YAP/TAZ are the evolutionarily-conserved key effectors of the Hippo pathway. Yki and YAP/TAZ are believed to mediate the biological functions of the Hippo pathway by regulating gene transcription. As transcriptional co-activators, Yki and YAP/TAZ cannot bind DNA directly, and they must interact with DNA-binding transcription factors to regulate target gene expression. In and different mammalian cell types have been profiled by independent studies. However, the overlap between these different gene profiling studies is not high, suggesting NIK that YAP/TAZ and Yki may regulate target gene expression in a tissue or cell type-specific manner. In and Hpo or MST1/2 are not absolutely required for regulation of Wts or LATS1/2. It has been observed that in mouse embryonic fibroblast (MEF) cells, MST1/2 double knockout did not abolish YAP phosphorylation, suggesting the existence of additional Hippo-like activity (Zhou et al., 2009). Indeed, a recent study in has identified Misshapen (Msn) as another kinase responsible for Wts activation. This mechanism is also conserved in mammals, as MAP4K4 (Msn ortholog) overexpression promotes phosphorylation of LATS1/2 (Li et al., 2014), and MAP4K4 knockdown induces activity of a YAP reporter (Mohseni et al., 2014). It is possible that additional kinases, especially some STE20 family members, may activate LATS1/2 in response to different upstream signals or in different tissue contexts (Figures 2 and 3). YAP/TAZ have also been shown to be phosphorylated by many other kinases such as cyclin-dependent kinase 1 (CDK1), Jun N-terminal kinases (JNK), homeodomain interacting protein kinases (HIPK), ABL, and Src family tyrosine kinases (reviewed in (Varelas, 2014)), suggesting that YAP/TAZ can be regulated by mechanisms independent of Hippo pathway kinases. Cell Polarity and Cell Adhesion Regulate Hippo Signaling In searching for upstream regulators of the Hippo pathway, many proteins involved in cell polarity and cell adhesion have been identified. Echinoid (Ed), a cell adhesion molecule in deletion lead to YAP activation and tumorigenesis (Cai et al., 2015). More studies may be needed to verify the mechanism of YAP/TAZ activation by Wnt. Epidermal growth factor (EGF) and insulin have also been shown to regulate YAP/Yki activity in cultured mammalian cells and gene (inhibits actin polymerization) in results in Yki activation and tissue overgrowth (Fernandez et al., K-Ras G12C-IN-1 2011; Sansores-Garcia et al., 2011). Similarly, knockdown of actin-capping proteins or filamentous actin (F-actin) severing proteins (cofilin or gelsolin) in mammalian cells also leads to YAP activation (Aragona et al., 2013). In general, Rho GTPase activity and F-actin appear to activate YAP/TAZ, whereas destabilization of F-actin inhibits YAP/TAZ (Dupont et al., 2011; Miller et al., 2012; Wada et al., 2011; Yu et al., K-Ras G12C-IN-1 2012; Zhao et al., 2012). Spectrin proteins, in association with short actin filaments, are organized into an elastic polygonal meshwork which lines the intracellular side of the plasma membrane. In epithelial cells, localization of the spectrin network is usually polarized and present in both apical and basolateral domains. In addition to a supporting role for cell structure, the spectrin network may transmit diverse signals from cell microenvironment to regulate cellular functions (Bennett and Gilligan, 1993). Recently, three independent studies revealed a regulatory role of the spectrin network on K-Ras G12C-IN-1 the Hippo pathway, and disruption of the spectrin network.