Based on previous studies that reported the secretion of Hsp90 by human breast cancer cells MDA MB 231 [42] and MDA MB 453 [21,41], lysates of MDA MB 231 and MDA MB 453 cells as well as concentrated supernatants derived from the serum-free cell medium upon 38 h of culture were analyzed by Western blotting using commercially available anti-Hsp90 antibody. The gene discussed is HSP90AA1; the disease is breast cancer.