Data from an animal model of mammary tumor virus (MMTV)-ErbB2 tumorigenesis underscore the preventive antitumor function of metformin, since it selectively inhibits the proliferation of a specific cellular subpopulation which is being incriminated for tumor initiation; those with the CD61(high)/CD49f(high) immunophenotype. This evidence concerns the gene ERBB2 and neoplasm.