If the gene is retroviral it can evolve from a retrotransposon into a retrovirus. Feline leukemia virus (FeLV) is a retrovirus that infects cats. PERVS were originally discovered as retrovirus particles released from cultured porcine kidney cells. The cells are negative for reverse transcriptase, which means that they lack integral retrovirus genomes. It was isolated from the murine retrovirus bearing the number 3611. SRC was first discovered as an oncogene in a chicken retrovirus. The virus was later called Rous sarcoma virus 1 and understood to be a retrovirus. This allows the retrovirus to implant nearby a cellular oncogene. They encode glycoproteins on the retrovirus envelope needed for entry into the host cell. The allele that codes for adenosine deaminase (ADA) was obtained and inserted into a retrovirus. It was the first vaccine to protect mammals from a retrovirus infection. The first retrovirus infecting humans was identified by Robert Gallo in 1974. An example of a retrovirus is HIV. It was suggested that ATL is caused by the infection of a retrovirus called ATLV. These studies established that the retrovirus infection is the cause of ATL.