Theories of cancer origin.

Throughout centuries, many theories have been proposed to explain the origin of cancer. In this review, some of them are presented and briefly commented upon. The theories discussed are divided into epigenetic and genetic ones, with the latter further classified into gene-based and genome-based hypotheses. Multistage cancer development leading to the mutator, malignant cell phenotype is also described. Finally, the current hot topic of cancer stem cells is presented.