DKK1 and esophageal squamous cell carcinoma: Testing of DKK‐1 and DKK‐1 autoantibodies together could differentiate ESCC from normal controls (area under the ROC curve [AUC] 0.769, 95% confidence interval (CI), 0.715–0.823, 50.3% sensitivity, and 90.7% specificity in the training cohort; AUC 0.752, 95% CI, 0.675–0.829, 50.0% sensitivity, and 84.9% specificity in the validation cohort).