The CEO of Be the Match, the Minneapolis nonprofit that manages the world’s largest bone marrow donor registry, stepped down this week citing “personal reasons.”

The organization, which has a $406 million budget and more than 1,000 employees, announced Tuesday that C. Randal Mills had resigned immediately. A spokesperson declined Thursday to elaborate on why he resigned. CFO Amy Ronneberg was named the nonprofit’s acting CEO until a permanent replacement is chosen.

Mills started the top job in 2017 and made $475,791 a year in compensation, according to 2018 tax forms. He previously was chief executive for the California Institute for Regenerative Medicine.

Be the Match moved from northeast Minneapolis to a seven-story headquarters near Target Field in 2015. The company had struggled financially before then, laying off 50 employees and cutting $18 million to adjust to a decline in the number of blood transplants, which bring in fees as its main revenue.

Be the Match, which is operated by the National Marrow Donor Program, started in Minnesota in the 1980s.