When the most types of mammalian cells are cultured in a dish in presence of serum, they adhere to the bottom on the dish and spread out until a confluent monolayer is formed. Each cell is attached to the dish and contacts its neighbours on all sides. At this point normal cells stop proliferation- this phenomenon was described as contact inhibition of cell division. Cancer cells usually disregard these restraints and continue to grow, so that they pile up on top of one another.
In a transfection experiment, DNA from a human bladder carcinoma is added to a culture of mouse 3T3 cells, causes about one cell in a million to divide abnormally and provide a distinctive phenotype. The progeny of this affected cell are more rounded and less adherent to one another, even to the dish where it was cultured. These transformed cells have many properties of a cancer cells such as, changes in cell morphology, ability to grow, unattached to a extracellular matrix, reduced requirement of growth factors, secretion of plasminogen activator and loss of actin microfilaments.