Two levels of genetic resistance to lymphoid leukosis.

Two levels of genetic resistance to lymphoid leukosis are recognized: 1) cellular resistance to virus infection; and 2) resistance to tumor development in leukosis-virus-infected birds. Resistance to infection is simply inherited but is very specific for the subgroup of virus. Inheritance of resistance to tumor development is more complex but appears to be less subgroup-specific. A breeder may wish to select for resistance to infection of virus eradication is the goal. If his goal is the reduction of lymphoid tumors, with virus infection not important, he may choose to select for resistance to tumor development.