Proof of in vivo fusion of murine tumor cells with host cells by universal fusers.

Two mouse cell lines (A9HT CI.3C and 501-1), each carrying both a recessive and a dominant mutation (and therefore designated "universal fusers"), were utilized for selection of in vivo hybrids from tumors produced intraperitoneally and subcutaneously in appropriated hosts. The double selective medium eliminated both parental (tumor and host) cells but allowed the survival and proliferation of the fused product. This proved that hybridization between tumor and host cells occurred in vivo.