Inverted U-shaped dose-effect curves were observed with regard to the effects of PTK7 expression on cell proliferation (Fig. 3c and d and Supplementary Fig. S1c and d), migration (Fig. 3e and f and Supplementary Fig. S1e and f), and invasion (Fig. 3g and h and Supplementary Fig. S1g and h) of the ESCC cells, with increases from low levels promoting these oncogenic properties and higher levels suppressing them. Here, PTK7 is linked to esophageal squamous cell carcinoma.