Cancer cells are those cells which have lost the usual control over their growth and division. They are characterized by immortalization (indefinite growth), transformation and metastasis (invasion of normal tissue). Metastasis really distinguishes a malignant tumour (ability to invade normal tissue) from a clinically benign (does not invade new tissue) tissue. In culture also, cancerous cells differ from the established cell lines, which are characterized by the dependence of their growth on the availability of a 'limited density' (growth is inhibited beyond this density). The cancerous cells do not need a solid surface for growth, have reduced dependence on serum, may form a thick mass of cells (instead of a thin layer) and induce tumours when injected into appropriate test animals. These cancer cells may be compared with normal cells to identify the genetic basis of tumour formation.
Tumour formation may be spontaneous, or induced by carcinogens or tumour viruses. It may also be inherited like Mendelian traits. A number of genes have now been identified in viruses and in the host cells that have the potential of causing tumour formation. They are called oncogenes (roughly five dozen oncogenes were already discovered by the end of 1990) and are described
as viral or v-onc genes (present in viruses) and cellular or c-onc genes (present in host cells). Often the genes present in host are not oncogenic and are, therefore, described as proto-oncogenes, which have the potential of becoming oncogenes. There are also genes which suppress tumour formation. These are called anti-oncogenes or tumour suppressor genes. A variety of these genes are now known and will be discussed in this section.