肿瘤或失去控制的细胞增生是由于突变使癌基因启动从而促使增生，或者由于肿瘤抑制基因和肿瘤抑制基因产物的缺失。

Neoplasia, or uncontrolled cellular proliferation, can result either from mutations that "turn on" the oncogenes that stimulate growth, or from mutations that result in loss of tumor suppressor genes and their products that inhibit growth.