In the present study, we examined the expression of major proteins (MST1, MST2, LATS1, LATS2, and YAP1) in the Hippo pathway in OSCC, carcinoma in situ (CIS), and its precursor lesion, oral epithelial dysplasia (OED), and investigated the clinicopathological significance of Hippo pathway protein expression in OSCC. The gene discussed is MST1; the disease is in situ carcinoma.