OPRK1 and esophageal squamous cell carcinoma: OPRK1 expression has been reported to be associated with a significantly poorer prognosis and tumor migration in various cancers, such as esophageal squamous cell carcinoma (ESCC) [10], and liver metastases of small bowel and pancreas neuroendocrine tumors [11], and these results strongly suggest an essential role of OPRK1 in tumor growth and migration.