CDKN1B and cancer: Given it is expected that approximately 1 × 109 cells are contained within 1 gram of tumor and that 1–10% could be cancer stem cell clonogens, a difference in a final cell kill between 1 × 109 versus 1 × 1015 would be the potential difference in tumor control (all clonogens killed) versus lack of control, based on p27 expression.