The gag-onc fusion protein is a general term for a fusion protein formed from a group-specific antigen (gag) gene and that of an oncogene (onc), a gene that plays a role in the development of a cancer. The name is also written as Gag-v-Onc, with "v" indicating that the Onc sequence resides in a viral genome. Onc is a generic placeholder for a given specific oncogene, such as C-jun. (In the case of a fusion with C-jun, the resulting "gag-jun" protein is known alternatively as p65).