Gene Transfer Experiment. Human tumor cell DNA encodes a cancer gene (oncogene) as shown by a gene transfer experiment. DNA was extracted from a human bladder cancer and introduced into "normal" mouse fibroblasts (a cell line that exhibits contact inhibition: cells stop dividing when they touch each other in culture). Cells that have lost contact inhibition continue to divide when they touch each other and pile up to form a focus (the cells in the focus are called "transformed" cells). "Transformed" mouse fibroblasts form a tumor when injected into a mouse. "Normal" mouse fibroblasts do not form a tumor when injected into a mouse. This diagram shows a single passage of the human tumor DNA through mouse fibroblasts. However, DNA from the transformed mouse fibroblasts can be used to transform mouse fibroblasts again and again in serial cycles. Even after multiple cycles, the transformed mouse fibroblasts (still expressing the dominant human oncogene) form tumors when injected into mice.