The MMTV-Hras/p53R172H/R172H tumors closely resemble the MMTV-Hras/p53-/- tumors with regard to most of these parameters, suggesting that loss of the sequence-specific DNA binding function of wild-type p53 is the primary cause of the difference in tumor histology between tumors with mutant and wild-type p53. This evidence concerns the gene HRAS and neoplasm.