Tumour suppressor genes, which determine the development of a cancer in some circumstances, have been discovered more recently than oncogenes. It appears that there are within cells many control elements designed to suppress the development of a cancer. They work by acting as negative controls on the process of cell proliferation and the name 'tumour suppressor gene' is probably the best one, although they are frequently referred to as anti-oncogenes. Tumour suppressor genes are harder to study than oncogenes simply because they act as suppressors. When they are doing their work, mere are no cancerous changes to study. A tumour suppressor gene has to be deleted from a cancer cell before the cancer pattern can emerge. Examples of tumour suppressor genes are still relatively few but many more probably remain to be discovered. The best known is a gene that suppresses the development of a rare kind of cancer called retinoblastoma which develops in the eyes of children. This is one of the few cancers that clearly runs in families and it does so because abnormal versions of a particular tumour suppressor gene are inherited from parents by their children. The normal cell contains two copies of the tumour suppressor gene and inheriting a single non-functioning copy from parents is not enough to cause the cancer. The other gene has to be inactivated by a process that occurs after the birth of a child. When both copies are inactive, the cancer develops. In the development of commoner cancers it is unusual for inheritance of an abnormal tumour suppressor gene to be the important mechanism and it is probable that several events must occur after birth before the tumour suppressor genes are fully inactivated. Although these genes were initially discovered as a result of studies carried out in inherited cancers, we now believe that abnormalities in these genes can occur without any inheritance from parents.
The most important tumour suppressor gene known to us so far is probably one called P53, discovered by Professor David Lane at the Imperial Cancer Research Fund in London. It is the gene which is most commonly found to be abnormal in any cancer.