This model suggests that immunological properties may be important for understanding tumorigenesis and heterogeneity. As such, CSCs can be very rare in some tumors,[14] but some researchers found that a large proportion of tumor cells can initiate tumors if transplanted into severely immunocompromised mice,[15] and thus questioned the relevance of rare CSCs. However, both stem cells[16] and CSCs[17] possess unique immunological properties which render them highly resistant towards immunosurveillance.