A prominent cancer doctor who referred asbestos-disease patients to former Assembly Speaker Sheldon Silver's personal injury law firm testified Wednesday he used the potentially lucrative cases to curry favor in hopes of getting research funding, but did not say that Silver ever explicitly linked the two.

"I wanted to maintain a relationship with him so that he would be incentivized to be an advocate for mesothelioma research and he would help us with funding for mesothelioma research," said Dr. Robert Taub, the star government witness at Silver's corruption trial in federal court in Manhattan.

But during cross-examination, the bow-tied former Columbia University professor seemed conflicted about the arrangement, calling Silver a "friend" and telling a defense lawyer the mesothelioma research funding Silver pushed was "absolutely the right thing" and funded "good work for New Yorkers."

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And when Silver lawyer Steve Molo asked if Taub and Silver had "an explicit agreement to exchange patients for grants," Taub responded, "I did not."

Taub's testimony came on the second day of Silver's trial on bribery, extortion and money-laundering charges. The longtime Albany power broker is accused of giving two $250,000 state research grants to Taub in return for patient referrals, and doing legislative favors for developers in a similar scheme.

Silver, 71, resigned after two decades as speaker when he was charged in January by U.S. Attorney Preet Bharara but continues to represent Manhattan's Lower East Side. The case, along with the upcoming trial of former Senate leader Dean Skelos, is viewed as a linchpin of Bharara's crusade against Albany corruption.

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Taub, 79, a widely regarded expert on mesothelioma who established a research foundation at Columbia, said he had devoted his life to helping those with the "terrible" disease, but was fired when the Silver charges became public. He said he was suing Columbia.

Silver and Taub first met in 1984, he testified, through a common friend, one-time Assembly aide Daniel Chill, and Taub approached Silver in 2003 because he had joined a major New York law firm for asbestos victims -- Weitz & Luxenberg -- that didn't donate to asbestos research, and he wanted to see if Silver could change the policy.

Silver said he couldn't help, but "one to two weeks later" Taub said he learned -- in a conversation with Chill that he was not allowed to detail after a hearsay objection -- that Silver wanted referrals.

Taub said he knew the cases were "valuable," generating legal fees of one-third on multimillion-dollar recoveries, and felt Silver -- who got one-third of the firm's fee on referrals -- would make sure his patients were taken care of.

"I hoped to develop a relationship with him that would help fund mesothelioma research, and would help my patients," Taub testified. At some point, he said, Silver "conveyed that he was pleased with the referrals he was getting."

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"Several months" later, Taub testified, he learned through Chill that Silver wanted him to apply for state research funding. He drafted a letter describing the research and requesting money. Prosecutors introduced an Assembly copy of it with a handwritten notation, "Shelly is very interested in this."

Taub said he got $250,000 grants in 2005 and 2006, with less than the normal "vetting." Silver, he said, turned him down for a third grant in 2007, telling him, "I can't do this anymore." The indictment says the grants stopped because the health funds Silver was using ran out.

Over 10 years, Taub testified, he referred 25 to 50 cases to Silver, calling his Assembly office to give contact information on patients. Although he never testified to a conversation linking grants and referrals, Taub did say Silver had a "pattern" of expecting favors to be repaid.

He recalled a conversation in which Silver asked him to keep the arrangement private.

Referrals to Silver shrunk after Taub started giving cases to a law firm that donated to his research, he said, and Silver at one point asked about the reasons. They remained on friendly terms until 2014, when Taub was approached by investigators and agreed to testify under a grant of immunity.

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In addition to the grants, he also testified about other favors Silver did -- helping Taub's children get work, offering to clear red tape for a charity event and arranging an Assembly citation for Taub's work. He said the favors were because they were "friendly," but acknowledged the relationship was subtle.

"If you ask him to do something, he would ask for something in return," he testified. "If it's family or a very close friend, that's not what you expect."

Taub's testimony is expected to resume Thursday.