OPRK1 and cancer: Similarly, expression of OPRK1, an opioid receptor that belongs to the superfamily of G protein-coupled opioid receptors, has been reported to be associated with a significantly poorer prognosis and tumour migration in various cancers, such as breast, oesophageal squamous cell carcinoma, metastatic liver cancer, and pancreas neuroendocrine tumours [57,58].