Tumour suppressor gene
Tumour suppressor genes code for proteins that inhibit cell division, repair DNA or initiate cell death.
If a tumour suppressor gene mutates, the functional protein is no longer produced and the above processes no longer occur. As a result, the cell survives and carries on dividing when it should not.
Tumour suppressor gene mutations are classified as loss of function mutations. This means that the gene's function is either lost or reduced. The protein the tumour suppressor gene codes for may become less effective, or it may not be produced at all.
If proto-oncogenes are the "green light", allowing the cell to divide, tumour suppressor genes are the "red light", preventing cell division.
Loss of function mutations are normally recessive mutations, meaning that two mutated copies must be present for an effect on cell function.
One functioning tumour suppressor gene can still produce enough protein for the cell cycle to continue normally.