PKP1 and esophageal squamous cell carcinoma: Many studies have suggested that the expression of PKP1 was significantly decreased or deleted in malignant tumors, including oral/pharyngeal squamous cell carcinoma, esophageal squamous cell carcinoma, prostate cancer [30–32], and high expression of PKP1 inhibited proliferation, migration and invasion of these tumors, suggesting that PKP1 may be served as a tumor suppressor gene.