A common aspect of tumour formation is deregulation of normal cell-cycle controls.
Normal cells are subject to both positive and negative growth stimulii.
These are finely balanced during development and adulthood to ensure growth or homeostasis when appropriate. Tumour cells have acquired genetic changes which disturb this balance resulting in inappropriate growth. Ras transformed cells are able to proliferate in the absence of mitogens, in the absence of anchorage and are immune to the growth inhibitory effects of normal surrounding cells. These effects on the cell-cycle are likely to be central to the role of Ras in the tumourigenic process. In these more normal cells further genetic changes are required for Ras to be able to overcome cell-cycle controls.