Normal cells stop growing when they contact other cells, eventually forming a layer of well-ordered cells (Figure 24-5a). Cancer cells are less adherent, forming a three-dimensional cluster of cells (a focus) that can be recognized under a microscope (Figure 24-5b). This loss of confinement to tissue structures can be modeled in vitro. Cultured mouse fibroblasts called 3T3 cells normally stop growing when they contact other cells, eventually forming a monolayer of well-ordered cells that have stopped proliferating and are in the quiescent G₀ phase of the cell cycle (see Figure 24-5a). When DNA from human bladder cancer cells is transfected into cultured 3T3 cells, about one cell in a million incorporates a particular segment of the exogenous DNA that causes a phenotypic change (see Classic Experiment 24-1). The progeny of the affected cell are more rounded and less adherent to one another and to the culture dish than are the normal surrounding cells (see Figure 24-5b). Cells that do not cease division when they contact other cells are said to be no longer “contact inhibited” and to have undergone oncogenic transformation. Recent work has implicated adhesion molecules such as E-cadherin, cell polarity factors, actin cytoskeleton regulators, and the Hippo pathway in mediating cell cycle arrest when cell-cell contacts are established. However, the exact mechanisms whereby this occurs, and how these pathways are disrupted in cancer, remain to be worked out.