In a healthy cell, tumor suppressor genes act as the gatekeepers that control cell growth and proliferation and promote cell apoptosis. Hence, when a tumor suppressor gene is inactivated, cells may start growing out of control leading to cancer. Fortunately, in most cases, the inactivation of a single allele of a tumor suppressor gene does not lead to tumor progression. Both of the alleles must lose their function for the cells to become cancerous. Such loss of function mutations in tumor suppressor genes can be caused by genetic alterations such as point mutations, chromosomal deletion, or errors in chromosome segregation. However, a few epigenetic changes can also permanently inactivate the tumor suppressor genes. For example, if a gene undergoes modifications in its promoter region or gets packaged into the heterochromatin region of a chromosome, it can make irreversible and heritable changes to the gene expression pattern.