HSP90AA1 and breast cancer: Hsp90α is encoded by the gene Hsp90aa1; for clarity, we will refer to it here as Hsp90α. To study the effects of its deletion on mammary tumorigenesis, we compared mammary tumor initiation and progression in mice with the PyMT transgene on the two genetic backgrounds (Hsp90α+/+ versus hsp90α−/−).