A tumor suppressor gene, or antioncogene, is a gene that protects a cell from one step on the path to cancer. When this gene mutates to cause a loss or reduction in its function, the cell can progress to cancer, usually in combination with other genetic changes. The loss of these genes may be even more important than proto-oncogene/oncogene activation for the formation of many kinds of human cancer cells. Tumor suppressor genes can be grouped into categories including caretaker genes, gatekeeper genes, and landscaper genes; the classification schemes are evolving as medicine advances, learning from fields including molecular biology, genetics, and epigenetics.