The basic function of tumor suppressor genes is to restrict cell growth.
When tumor suppressor genes are inactivated (e.g by gene mutation), cells grow without restraint, which allows cancer to develop.

Approximately 50 genes have been presumed to belong to the substantial class of Tumor Suppressor and Repair Genes. Among these, p53 seems to be of special importance:

A defect in the p53 pathway is either directly or indirectly involved in the majority of human cancers.