I n 1981, groups led by Robert Weinberg,

discovered that small fragments of DNA taken from human cancer-derived cells could endow malignant characteristics on normal mouse fibroblast cells. Within a year, the same groups, along with that of Mariano Barbacid, established that the active ingredient in transforming DNA from a human bladder cancer cell line was the cel-lular homolog of H-RAS, an oncogene found in the Harvey rat sarcoma retrovirus. They thus demonstrated for the first time that human tumors contained activated onco-genes, related to those picked up by retro-viruses from their host genomes. Within a few months, the same groups further foun