Herbimycin A, a benzoquinoid ansamycin antibiotic, irreversibly and selectively inhibits tyrosine kinases by reacting with thiol groups (1,2). It is effective on Src, Yes, Fps, Ros, Abl and ErbB oncogene products (1-3) and inhibits thrombin-induced tyrosine phosphorylation of phospholipase C (4). Looking for more information on HSP90? Visit our new HSP90 Scientific Resource Guide at http://www.HSP90.ca.