1. a term applied to a group of transmissible virus-induced diseases of chickens causing sarcoma, myeloblastosis, erythroblastosis, leukosis, osteopetrosis, and lymphomatosis. These agents are closely related viruses (avian leukosis-sarcoma virus) causing prolferation of immature erythroid, myeloid, or lymphoid cells;
2. a division of the RNA tumor viruses (subfamily Oncovirinae) causing the avian leukosis-sarcoma complex of diseases; the viruses are subgrouped according to antigenic characteristics and growth in defined types of tissue culture cells. Syn: avian erythroblastosis virus, avian leukosis-sarcoma virus, avian lymphomatosis virus(1), avian myeloblastosis virus, avian sarcoma virus, fowl erythroblastosis virus, fowl lymphomatosis virus, fowl myeloblastosis virus.