HRAS and neoplasm: To determine whether a p53 gain-of-function mutation further accelerates the tumor growth rate, the growth of multiple independent salivary tumors arising in mice of each of the three different genotypes (MMTV-Hras/p53+/+, MMTV-Hras/p53-/-, and MMTV-Hras/p53R172H/R172H) was compared (Fig. 2; S1 Fig.).