Genetic instability of cancer. Why a metastatic tumor is unstable and a benign tumor is stable.

It is theorized that tumors may be initiated by two methods: by an error affecting one or several oncogenes, or by an error affecting one or several of the genes controlling the stability of the genome. The majority of cells that misexpress an oncogene(s) and that later form a tumor probably form nonevolving benign tumors. A minority of these cells with an activated oncogene(s) (or one of the descendant cells) may also come to misexpress a stability gene(s). A normal cell that misexpresses only a stability gene(s) may form an evolving and genetically unstable cell line that may later misexpress an oncogene(s). A cell or cell line that misexpresses both an oncogene(s) and a stability gene(s) may form a genetically unstable tumor that creates diverse variants, allowing for extensive tumor cell evolution and the acquisition of malignant and metastatic properties.