Dr. Mehmet Oz, the celebrity physician nominated by President Donald Trump to oversee Medicare and Medicaid, appears to have underpaid his U.S. Social Security and Medicare taxes in recent years, according to a memo by Democratic staffers on the Senate Finance Committee.

Trump’s allegiance to Oz was further emphasized on Tuesday when the president announced he was nominating Oz’s son-in-law, John Jovanovic, to head the Export-Import Bank of the United States.

Television personality and surgeon Dr. Mehmet Oz is scheduled to appear on Friday before the Senate Finance Committee, on which Warren sits. The panel will hold a confirmation hearing for his nomination to be administrator of the Centers for Medicare and Medicaid Services,

Sen. Elizabeth Warren (D-Mass.) has written a letter to Mehmet Oz, asking him to make a series of commitments to avoid conflicts of interests ahead of his confirmation hearing Friday to lead the Centers for Medicare & Medicaid Services (CMS).