Avian leukosis virus (ALV)
Avian leukosis viruses (ALV) are a group of retroviruses that can induce tumors in poultry. ALVs are simple, RNA viruses that are classified into different subgroups based on virus-specific cellular receptors and virus envelope glycoproteins. Chickens are affected by subgroups A-E and J. ALVs are characterized by possession of the enzyme reverse transcriptase, required for the formation of a DNA provirus. This provirus is integrated into the host genome during viral replication. Certain breeds of chickens are genetically resistant to ALVs.