Histological study of breast cancer specimens obtained from 294 female residents in the Republic of Sakha (Yakutia) of different age and ethnic groups and immunohistochemical analysis of protein expression of mutant suppressor gene p53 in tumor cells were performed. A positive correlation was found between expression of mutant gene p53 protein and the degree of malignancy of breast tumors. The indicator should be considered when assessing malignancy potential and risk of breast cancer, because it is important for optimization and individualization of the treatment.