PTK7 and breast cancer: On the other hand, it has been shown that PTK7 is downregulated in several types of cancer, including clear cell renal cell carcinoma, metastatic melanoma, pulmonary adenocarcinoma, epithelial ovarian carcinoma, esophageal squamous cell carcinoma and breast cancer cells, indicating that PTK7 can function as a tumor promotor or tumor suppressor in different tumors or organs by regulating cell proliferation, migration, invasion, or apoptosis [43,44,45]).