HSP90AA1 and breast cancer: At this point it should be noted that preliminary experiments performed using the MDAMB231 breast cancer cells which lack the HER2 receptor, indicate not only that these cells too secrete both isoforms of HSP90 but also that MMP2 and MMP9 activation by extracellular HSP90 may be independent of the HER2/HSP90 interaction previously reported [14], since in this system as well mAb 4C5 prevented activation of these metalloproteinases (unpublished data).