Cancer of unknown primary origin (CUP) is a term used to describe a cancer that is determined to be at the metastatic stage at the time of diagnosis, but a primary tumor cannot be identified. A diagnosis of CUP requires a clinical picture consistent with metastatic disease and one or more biopsy results inconsistent with a primary tumor.CUP is found in about 3 to 5% of all people diagnosed with invasive cancer, and carries a poor prognosis in most (80 to 85%) of those circumstances.