Shares of biotech



plunged more than 40% and trading was halted after it was learned of a regulatory delay in approval for one of its drugs.

Tanox was off $10.29, or 40.6%, to $15.07 when trading was stopped shortly before 11 a.m. EDT. The company's asthma drug Xolair, to be marketed with partners


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Novartis AG

, was the subject of a letter from the

Food and Drug Administration

requesting more information.

As a result of the request, Genentech and Novartis said the drug would likely not be resubmitted until 2002 or 2003. In recent trading, Genentech was off 12%.