Tumor suppressor genes
["anti-oncogenes" (Knudson 1971); "recessive oncogenes"; growth inhibiting genes]

Tumor suppressors are genes in which loss of function is associated with tumor formation. Tumor suppressor genes and their protein products generally impose some constraint on the cell cycle or on cell growth.
Mutations in tumor suppressor genes that are associated with tumorigenesis generally cause loss of function and release this constraint.

Tumorigenesis associated with mutations in a tumor suppressor gene generally requires loss of both alleles at a locus.

• In inherited cancer syndromes one germline mutation in the tumor suppressor gene is inherited. A second, sporadic mutation of the other allele in a cell can then lead to initiation of cancer.
• In the sporadic form of the disease, both parental alleles are normal, and both copies of the allele are deactivated by two separate (somatic) mutational events in a single cell.

Missense Mutation

A missense mutation refers to a type of point mutation (substitution of a single nucleotide in a triplet codon by another nucleotide) that causes a substitution of the wildtype amino acid in the corresponding protein by another amino acid.

Transversion

A transversion is a type of point mutation (subsitution of a single nucleotide) in which a purine (adenine or guanine) is replaced by a pyrimidine (cytosine or thymidine) or a pyrimidine is replaced by a purine.

Transition

A transition is a type of point mutation (subsitution of a single nucleotide) in which a purine (adenine or guanine) is replaced by another purine or a pyrimidine (cytosine or thymidine) is replaced by another pyrimidine. In general, transitions are more common than transversions.