OPRK1 and hepatocellular carcinoma: Although 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], the downregulation of OPRK1 in hepatocellular carcinoma (HCC) tumor tissues has a strong association with poor prognosis and OPRK1 might be a potential tumor suppressor [36].