In order to validate the presence of extracellular Hsp90 in our cell lines, the surface proteins of the breast cancer cell lines MDA-MB-231, MCF-7 and Hs578T were labelled by biotin tagging (using cell impermeable NHS-biotin) and isolated using biotin-streptavidin affinity purification. The gene discussed is HSP90AB1; the disease is breast carcinoma.