A transmissible avian neoplasm (sarcoma of the common fowl).

Original description of the chicken sarcoma (Rous sarcoma). Rous demonstrated that sarcomatous tumors in hens could be transmitted to normal hens by the injection of cell-free filtrates (virus) of the original tumor. The Rous Sarcoma Virus was the first oncovirus discovered.

Fifty-five years later, in 1966 Rous shared the Nobel Prize in Physiology or Medicine with Charles Huggins "for his discovery of tumor-inducing viruses and his work on cancer."