VIM and breast cancer: Vimentin has never been described in the non-neoplastic luminal epithelium of the mammary gland of any species, however, it has been described in the so-called ‘cap cells’ of the mice and in the “side-population” of human breast containing the progenitor cell compartment, further validating a potential role of early breast progenitor cells in the pathogenesis of vimentin-expressing breast cancers [45].