KLK4 and esophageal squamous cell carcinoma: Four proteins from kallikrein-related peptidase family, namely, hK8(kallikrein-8), hK11(kallikrein-11), KLK13(kallikrein-13) and hK14(kallikrein-14), were detected by Olink Oncology II panel, and we found all of them had low levels in serum in ESCC patients regardless tumor stage, compared with healthy controls.