Still, Pfizer Inc. announced last week that it has signed a license with the Wisconsin Alumni Research Foundation to use human embryonic stem cells in the development of new drugs.

 WARF controls many of the worldwide patents for embryonic stem cells that companies or universities may use in research. While adult stem cells have been used to treat patients with more than 100 different diseases or conditions, embryonic stem cells, when used in animals, have tended to produce tumors. They have also triggered a negative immune system response and have been rejected when injected as a therapy.

But Pfizer is now the biggest of 35 companies to sign an embryonic stem cell license with WARF, according to Andy DeTienne, WARF's licensing manager for stem cells.

Ruth McKernan, Pfizer Medicine's chief scientific officer, said in a statement that the drug company plans to use the embryonic stem cell lines to "explore a whole new range of therapies" including creating specialized human tissue to regenerate damaged tissue.

In November, Pfizer launched a regenerative medicine unit that will focus on working with human embryonic stem cells. WARF patented embryonic stem cells after Wisconsin-based researcher James Thomson discovered them and the patients have been controversial ever since.

Last year, the U.S. Patent and Trademark Office rejected challenges from two foundations against the validity of the patients and WARF has seen more interest in the embryonic cells since that decision.

Another WARF licensee, California-based biotech firm Geron, was given approval last year from the FDA to be the first to begin clinical trials of a supposed therapy in humans based on embryonic stem cells, although there is question of whether actual human embryonic stem cells will be used in the trials or derivatives.