In support of this view, it has been shown in mouse models that the propensity for breast cancer cells to divide symmetrically and self-renew is affected by the mutational state of ErbB2 and p53 and that the relative ratio of symmetric-to-asymmetric tumor-propagating cell divisions may change over time in response to the tumor microenvironment (Cicalese et al. 2009). The gene discussed is TP53; the disease is breast cancer.