Director of the US Patent and Trademark Office Michelle Lee resigned today, without explanation.

Lee, who has been director of the office since 2014, was a favored candidate by the tech sector, who thought she brought a balanced approach to patents. She is a former Google lawyer and was one of the first corporate lawyers to speak out about the problem of so-called "patent trolls." Lee took office at a time when the tech sector was at odds with the pharmaceutical lobby and trial lawyers over a proposed patent-reform law, which did not end up passing.

Leadership at the USPTO has been unclear since the Trump administration transitioned into power in January. Early news reports said that Lee, an Obama appointee, would remain. But that was followed by a period of a few months in which the USPTO wouldn't even confirm who was director.

In April, more than 50 companies, including Facebook and Cisco, sent a letter to President Trump asking him to keep Lee on board, believing the office would apply stricter standards to technology and software patents under her leadership.

The USPTO director is nominated by the president and confirmed by the Senate. There's no telling how long the position will take to fill. The Trump administration still has not submitted nominees for 431 positions that require Senate appointment, according to The Washington Post. The lack of nominees, combined with a somewhat slower confirmation process in the Senate, has resulted in a far slower staff-up than any recent presidency.

"I am confident that the leadership team in place will serve you well during this transition," Lee told her colleagues in a memo that was obtained by the Post.

The newspaper also reported a message Lee wrote on her Facebook page, saying: "It has been a special honor to be the first woman to hold the position of Under Secretary and Director of the USPTO in our country's 200+ year history."

Lee didn't say what the next steps are for her. The White House didn't respond to a request for comment.

Update, June 7: USPTO Associate Solicitor Joseph Matal has been tapped to lead the office on an interim basis.