The consumer advocates contend the patents require any researcher in the United States working with embryonic stem cells – whether at a university, private research lab or drug discovery company – to pay exorbitant fees and adhere to such restrictive guidelines that researchers are effectively being “handcuffed.”

In a letter to the Wisconsin Alumni Research Foundation WARF, the patent office said it found the consumer groups raised “substantial questions” as to whether all three patents on embryonic stem cells should have been issued.

“We're pleased the PTO has decided to re-examine these patents,” said John M. Simpson, of the Foundation for Taxpayer and Consumer Rights in Santa Monica. “The patents should never have been issued in the first place.”

Joining in the request for a re-examiniation is the New York-based Public Patent Foundation and Jeanne Loring, a stem cell researcher at the Burnham Institute in La Jolla.

Officials at the Wisconsin foundation could not be immediately reached for their response.

Although there are 1.5 million patents active in the United States, the patent office receives only 400 to 500 request annually to re-examine patents, said Brigid Quinn, a spokeswoman for the office. A re-examination can result in no change to the patent or a dismissal of all or part of a patent.

She said about 90 percent of re-examination requests are granted.

Third party requests for patent re-examination, like the ones filed challenging the Wisconsin patents, are successful in having the patent either changed or completely revoked roughly 70 percent of the time, Simpson said.