Rous sarcoma

 A retrovirus that causes muscle or connective tissue cancer. It carries an oncogene named v-src. The protein v-Src and a protein found in chicken muscle cells - c-Src have very similar amino acid sequences, but c-Src is not an oncogene. The difference between c-Src and v-Src is that the chicken proteins' C-terminal has a key tyrosine residue near it. An interaction between said tyrosine residue and an upstream SH2 domain keeps the kinase domain inactive to stop uncontrolled cell growth. On the other hand, v-Src lacks the key tyrosine residue, which makes the protein always promotes cell growth, which can lead to cancer.