The in vivo malignant transformation of mouse fibroblasts in the presence of human tumour xenografts.

During the routine serial passage of over 30 human tumour xenografts in athymic (nu. nu.) mice over a period of 6 years the induction of murine fibrosarcomas at the site of transplantation has been observed on three occasions. In two cases it has been possible to follow the development of these tumours over successive transplant generations. These sarcomas had growth rates, tumour karyotypes and isoenzyme patterns which clearly distinguished them from the original human xenografts.