Two theories for carcinogenesis (cancer formation) have classically competed for medical validity. The first is the stochastic model, which theorizes that a group of cells were transformed by random events and began tumor formation. The hierarchical model, on the other hand, says that a small portion of cells produces a hierarchical organization not unlike stem cells, where cancerous cells reproduce and differentiate. [5]

It has been recently discovered that solid tumors such as breast cancer and colon cancer contain these stem-cell-like cancer cells, termed cancer stem cells (CSC)s or tumor inducing cells (TIC)s, providing more evidence that the hierarchical model is correct. This theory also provides an explanation for the recurrence of tumors even after treatment. However, there is still some debate that it is valid because not all cancers appear to follow the hierarchical model. Because we have determined that some cancers do follow such a model, we will gain much knowledge about the others that do because of the advancements in stem cell research. [5]