In breast cancer cells, Hsp90 is also essential for the stability and function of steroid hormone receptors [36], as well as the membrane receptor tyrosine kinase ErbB2 [37], whose enhanced expression correlates with malignancy of breast cancer progression [25] and which might also play an important role in carcinogenesis of canine mammary gland [38-41]. This evidence concerns the gene HSP90AA1 and breast cancer.