Certain retroviruses (members of the Genus Alpharetrovirus, Family Retroviridae) have the capability of causing erythroid leukemia (leukosis) in birds (Figure 71). They invade avians’ erythrocytes, introduce permanent changes of their genome, and consequently induce the development erythroleukemia in some of the affected cases. (In fact, erythroid leukemia was the first leukemia to be recognized as caused by a virus [Ellermann and Bang 1908]. The latter investigators, a physician and veterinarian respectively, transmitted erythroleukemia and myelogenous leukemia by inoculation of chickens with cell-free, virus-containing filtrates [Payne 1992].) These retroviruses are collectively entitled the avian leukosis/sarcoma viruses (ALSV or LSV) and are etiologic agents of erythroid, lymphoid and myeloid leukoses as well as some other non-hematologic neoplasms (Andrewes 1989, Ritchie 1995, Payne 2001). Like all retroviruses, they are RNA viruses that replicate via a DNA proviral stage linearly present in the host genome. The latter activity is made possible by virtue of a pol gene that is included in the viral genome and encodes the enzyme reverse transcriptase. This enzyme enables the transcription of viral RNA to DNA (Payne 2001). The viral genome consists of a single stranded RNA. Virus infection of the cells occurs through receptors on the avian cell membrane. Viral RNA is released in the cytoplasm where under the influence of reverse transcriptase, a strand of viral DNA is synthesized on the template of viral RNA. A second strand of viral DNA is formed to produce linear double-stranded viral DNA, which in turn, is transferred from the cytoplasm to the nucleus. This new viral DNA is integrated into a host chromosome as a sequence of units termed a provirus. The proviral DNA is dealt with by the cell as a cellular gene(s). It is duplicated with each mitotic division and thus remains integrated in the DNA of daughter cells. The infected cell transcribes the proviral genes, and synthesizes the proteins they encode. The products are assembled into new virions at the cell membrane and released from the cell by budding (Dougherty 1987). Diagrammatic representation of a retrovirus. Retroviruses are spherical, enveloped particles, 80–120 nm in diameter and have an icosahedral core (polyhedron with 20 faces). The envelope has external knobs or spikes on its surface composed of two proteins, of which at least the larger is glycosylated. The helical virion RNA is a 60–70S dimer of two identical units. The RNA has at least three genes: <italic>gag</italic> (group specific antigen) coding for 3–5 internal virion proteins, <italic>pol</italic> (polymerase) coding for a RNA-dependent DNA polymerase (a DNA polymerase enzyme that transcribes single-stranded RNA into double-stranded DNA, synonym: reverse transcriptase), and <italic>env</italic> (envelope) coding for proteins of the viral envelope. Andrewes Viruses of Vertebrates. 1989. J.S. Porterfield, editor. Baillere Tindall. London, Philadelphia. With permission from Elsevier. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429062162/8ae66357-2c60-47f4-8671-989c5c5a4525/content/fig71_B.tif"/>