tumor suppressor gene
/ TOO-mer suh-PRES-er jeen /
If a proto-oncogene is the cell's gas pedal, a tumor suppressor gene is its brake. A car needs brakes as much as it needs a pedal — they keep it from running wild. In the cell, tumor suppressor genes are the genes whose job is to slow down or stop division, repair damaged DNA, or, when a cell is too far gone, order it to safely destroy itself. They are the body's built-in restraint against cells dividing when they should not.
More precisely, a tumor suppressor gene codes for a protein that puts the brakes on the cell cycle, watches for DNA damage, or triggers the cell's self-destruct program (apoptosis). Unlike oncogenes, which act dominantly (one faulty copy is enough), tumor suppressors usually behave recessively: you carry two copies, and both must be knocked out before the brake is fully lost. This is the heart of Knudson's 'two-hit' idea — one damaged copy may be harmless because the other still works, but losing both leaves the cell with no brake at all.
Tumor suppressors matter enormously because their loss removes a safeguard rather than adding a drive — and a missing brake is harder to fix with drugs than a stuck pedal is to block. Famous examples include p53 (the 'guardian of the genome'), the Rb protein, BRCA1 and BRCA2 (linked to breast and ovarian cancer), and APC (colon cancer). A common misconception is that inheriting a faulty tumor suppressor copy means you definitely get cancer; in fact it raises the risk because you start life one hit closer, but the second hit still has to happen.
A person who inherits one faulty copy of the BRCA1 tumor suppressor still has a working second copy in every cell. But because they start one hit closer to losing the brake entirely, their lifetime risk of breast and ovarian cancer is much higher — which is why such mutations are weighed carefully in screening and prevention.
Tumor suppressors are brakes; usually both copies must fail.
Inheriting one faulty copy raises risk but does not guarantee cancer — the second copy must also be lost before the brake fully fails.