[Stem cell--is there any role in tumorigenic activity]

Stem cells are a quintessential key to proper behavior of homeostatic processes. They are often thought of as the solution to a wide range of human conditions, with the ability to rescue malfunctioning or non-functioning organs and tissues. However, there is increasing evidence that stem cells can play a central role in disease. Most recently stem cells have been implicated in cancer after not responding to homeostasic controls such as proliferation and differentiation. Cancer has long been seen as a disease that arises from mutations that impair the capacity of any cell within the organism to respond to the signals that regulate proliferation. Besides their scarcity or abundance, a second important issue with respect to cancer stem cells is their origin. A new defining model for carcinogenesis, the "cancer stem cell hypothesis" was put forward in 2006, according to which cancer is a stem cell disease that places malignant stem cells at the centre of its tumorigenic activity as they have the capacity to undergo self-renewal, and have the potential to differentiate into different types of cells in a specific lineage.