Spontaneous fibrosarcoma in a thymus-transplanted nude mouse

A fibrosarcoma was observed in a 6-month-old nude mouse that had a neonatal thymus transplant and had been kept since birth under nearly pathogen-free conditions. This was the first tumor found in more than 1,000 nude mice maintained in a colony in which mammary carcinomas were observed among the female heterozygous breeders. This finding confirmed the fact that malignant neoplasms are rare in an animal model that lacks T-cell-mediated immunologic capability and apparently contradicted the postulates of the immunosurveillance theory.