DKK1 and esophageal squamous cell carcinoma: Compared with above‐mentioned studies of serum DKK‐1 in the diagnosis of ESCC, our study is unique for the following respects: Firstly, we recruited 88 patients with early ESCC (52 in the training cohort and 26 in the validation cohort), which enabled us to assess the early diagnosis performance of serum DKK‐1.