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Former Assembly Speaker Sheldon Silver arrives to court in New York, Tuesday, May 3, 2016. The former New York Assembly Speaker's corruption conviction was overturnedby a federal appeals court on Thursday. (AP Photo/Seth Wenig)

ALBANY, N.Y. -- A federal appeals court on Thursday overturned the Sheldon Silver's 2015 corruption conviction for pocketing $4 million in kickbacks from a cancer researcher and real estate developers.

He was sentenced to 12 years in prison in May 2016. But he appealed earlier this year after the Supreme Court narrowed the definition of "official act" for the purposes of proving corruption cases. That Supreme Court case centered on alleged criminal conduct by former Virginia Gov. Bob McDonnell.

In Silver's case, the court ruled his jury was given a too-broad definition of what constitutes an "official act" in light of the Supreme Court's decision, according to the ruling.

Silver was the New York State Assembly speaker for more than 20 years and one of the most powerful politicians in the state when he was indicted on a charge brought by then-U.S. Attorney Preet Bharara.

He was convicted on two counts of honest services mail fraud, two counts of honest services wire fraud, two counts of extortion under color of official right, and one count of monetary transactions involving crime proceeds.

As speaker, he had the power to award or withhold grants worth millions of dollars. He controlled millions of dollars in campaign donations that he used to help elect candidates.

He was also an attorney for a Manhattan law firm.

He resigned as speaker Jan. 27, 2015, five days after the FBI arrested him in the public corruption case. Silver remained an Assembly member until the jury found him guilty.

Silver was accused of receiving $700,000 in payments from one law firm in exchange for using his official position to obtain recurring tax certiorari legal claims of two real estate developer clients with business before the New York State Legislature.

He was also accused of receiving $3 million from a second law firm in exchange for using his official position to obtain names of unrepresented patients with mesothelioma from a doctor, to whose research Silver steered $500,000 in state funds.

Sheldon Silver through the years 24 Gallery: Sheldon Silver through the years

At trial, the federal judge defined an "official act' as "any action taken or to be taken under color of official authority." But the Supreme Court, in considering the former Virginia governor's case, narrowed the definition to an "official act" to be "a question, matter, cause, suit, proceeding or controversy" involving "a formal exercise of governmental power," according to the appeals court decision.

Good-government group Common Cause immediately blasted the court's decision and said it has "every expectation that the people can win again at retrial."

"Let us be clear, the facts are not in question. Mr. Silver used his position in government to direct state resources into a scheme that paid out $4 million into his pocket,' Common Cause executive director Susan Lerner said in a statement."...This is yet another area where the Court has misinterpreted the law, setting legal standards which fly in the face of the public expectations."

Bharara, the former U.S. attorney, posted to Twitter that he is confident Silver's conviction will withstand the latest twist.