HSP90AB1 and breast cancer: In contrast, Hsp90 in normal cells exists as an uncomplexed species with low ATPase activity and low affinity to Hsp90 inhibitors.294 Further study demonstrates that the difference is caused by a distinctive portion of Hsp90 forming complexes in cancer cells with oncogenic partners, such as Bcr-Abl-Hsp90 complex in mutant B-RAF-Hsp90 in SkMel28 melanoma cells, K562 chronic myeloid leukemia cells, Her3-Hsp90 and Raf1-Hsp90 complex in MDA-MB-468 breast cancer cells.