In those diagnosed with CUP, ~20% fall into the ‘favourable prognosis’ group (i.e. single resectable metastatic site, extragonadal germ cell syndrome, neuroendocrine cancers, squamous neck or inguinal lymph nodes, axillary node adenocarcinoma in females, adenocarcinoma with colorectal phenotype, peritoneal disease in females with serous or papillary histology, bone metastases with high prostate-specific antigen (PSA) in males), which share clinicopathological characteristics with particular known metastatic cancers [3, 6, 7]. The gene discussed is KLK3; the disease is metastatic malignant neoplasm.