The transformed clonal isolates of Indian muntjac diploid cells by a mouse sarcoma virus, 43-2XV, were tested for tumorigenicity in athymic nude mice. In spite of the indistinguishable transformed morphology, the tumorigenicity exhibited four different patterns: (a) no tumor formation; (b) slowly growing regressive tumor formation; (c) rapidly growing regressive tumor formation; and (d) rapidly growing progressive tumor formation. This demonstrates that the same diploid host cells transformed by the same virus reveal variable patterns of tumorigenic expression and some transformed host cells lack the tumorigenicity entirely. The findings that there are at least two chromosomes and four recombinant sites assigned for the proviral integrations of the sarcoma gene into the Indian muntjac gene (M. Hatanaka, R. Klein, R. Kominami, T. Oikawa, H. Okabe, N. Tsuchida, E. C. Connors, and A. Carrano. Transformation of Indian muntjac diploid cells by the proviral integration of sarcoma gene of a mouse retrovirus. Manuscript in preparation.) lead us to propose a hypothesis that variable expressions of tumorigenicity under the neutral background of immune responses, may arise from variable integrations of the sarcoma gene into the host chromosome.

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