Hippo-Lats-Yorkie signaling regulates tissue overgrowth and tumorigenesis in 
Drosophila. We show that the Mst1 and Mst2 protein kinases, the mammalian Hippo 
orthologs, are cleaved and constitutively activated in the mouse liver. Combined 
Mst1/2 deficiency in the liver results in loss of inhibitory Ser127 
phosphorylation of the Yorkie ortholog, Yap1, massive overgrowth, and 
hepatocellular carcinoma (HCC). Reexpression of Mst1 in HCC-derived cell lines 
promotes Yap1 Ser127 phosphorylation and inactivation and abrogates their 
tumorigenicity. Notably, Mst1/2 inactivates Yap1 in liver through an 
intermediary kinase distinct from Lats1/2. Approximately 30% of human HCCs show 
low Yap1(Ser127) phosphorylation and a majority exhibit loss of cleaved, 
activated Mst1. Mst1/2 inhibition of Yap1 is an important pathway for tumor 
suppression in liver relevant to human HCC.