Evidence shows that the B blood group locus in chickens, which controls red cell antigens, is associated with tolerance of skin homografts. Three other blood group loci studied did not show this effect.

DNA of Marek disease virus (MDV) consists of two unique regions UL and US flanked by long inverted repeat regions TRL and IRL, and short inverted repeat regions TRS and IRS, respectively, similar to herpes simplex virus DNA. Comparison of restriction patterns between pathogenic and nonpathogenic MDV DNA was made to identify a region of viral DNA different between these two types of MDV, as it may be responsible for the tumorigenicity of MDV in chickens. The results indicated that BamHI-D and -H, located at the long inverted repeat regions TRL and IRL, were specifically expanded in nonpathogenic viral DNA. The location of the expanded region has been determined within 1.5 kilobase pairs of the Bgl I/Pst I fragment of BamHI-D and -H, close to the junction between the inverted repeat and the unique region. The possibility that a gene responsible for tumor induction may be disrupted by such expansion has been discussed.

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Summary A comparison was made of growth patterns (progression/regression) of tumours induced by different strains of avian sarcoma virus in two partially congenic inbred lines of chickens homozygous for different MHC haplotypes. In each instance studied, the ability to regress tumours was shown to be a dominant trait controlled by MHC‐linked genes. The results also demonstrate a difference in growth pattern between tumours induced by different strains of virus in an inbred line as well as different growth patterns of tumours induced by the same strain of virus in the two inbred lines. We conclude that the MHC‐linked resistance gene is part of a polymorphic genetic region and, in addition, that there is immunogenic heterogeneity of the viral gene product expressed on tumour cells induced by closely related viral strains which is relevant to tumour regression. We suggest that the product of the src gene, p60src, is a plausible candidate for the immunogenic target of the MHC‐linked rejection response.