PTK2 and esophageal squamous cell carcinoma: Cai et al. consistently found that SPP1 is markedly overexpressed in esophageal tumors (both EAC and ESCC) and associated with advanced stage and lymph node metastasis; functionally, SPP1 drives tumor cell migration and invasion by activating the focal adhesion kinase (FAK)–ERK signaling pathway [40].