More and more tips of the iceberg are emerging from some recent reports. But they are nothing surprising based on an insight derived from Huang's overview (doi:10.1016/j.semcdb.2009.07.003). Future studies will be more exciting when we have considered and tested the conception shown in the figure below (original here).

Tumor I and II are derived from so-called 'stem cells' in sense of their differentiation potency. Type I occurs when the differentiated cells fail to lose proliferation potential; while type II occurs when abnormal cell types are generated unexpectedly.

Tumor III and IV are derived from differentiated cells at each level. Type III occurs when the reverse process of differentiation fails to produce the expected progenitors; type IV occurs when the doomed terminal cells 'create' their own descendant cell types, regaining capability of proliferation.