We propose that Mycograb® binds to hsp90, inhibiting hsp90 chaperone functioning and resulting in the destabilization of key proteins including estrogen/steroid receptors, nitric oxide synthase, ras1, MAP kinase, Src, Erb-B2, HER kinases and EGFR. Overexpression of HER2 receptors are observed in malignancies such as breast cancer and reportedly have been associated with resistance to chemotherapeutic agents. Both maturing and fully mature forms of the receptor depend on hsp90 association for stability. Inhibition of hsp90 function down regulates AKT kinase and Src kinase which are non-receptor kinase. Therefore, Mycograb® may be of use in estrogen dependent and hormone independent breast cancers.