Genzyme General

(GENZ)

rose Thursday after the Food and Drug Administration approved its drug Fabrazyme for the genetic disorder Fabry's disease.

In recent trading Genzyme was up $3.11, or 8.6%, to $39.49.

The news was a blow to

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Transkaryotic Therapies

(TKTX)

, which is offering a rival drug, Replagal, for treatment of the disease. The two companies are competing for "orphan drug" status with the FDA. Transkaryotic was down 73 cents, or 11.4%, to $5.73.

Fabry disease is a rare genetic disease -- only about 2,000 patients suffer from it in the U.S. -- but treatment is expected to cost upwards of $165,000 per year, and that makes this a $330 million market opportunity in the U.S.

Both Fabrazyme and Replagal already are approved and marketed in Europe, but in the U.S., the FDA will choose one drug under rules defined by the Orphan Drug Act. The act grants a single drug approval for a disease and grants it seven years of market exclusivity.

Transkaryotic's drug was turned down by a FDA panel in January, which found there was not enough data to support approval. But the competition between the two drugs may not be over. If Transkaryotic can persuade drug regulators to accept a re-analysis of the data, it might clear the way for approval and a continued struggle between the two drugs.