U.S. President Barack Obama and his wife, first lady Michelle Obama, offered condolences on the death of Nancy Reagan on Sunday, saying she "redefined the role" of first lady and became a strong advocate for Alzheimer's research.

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"Nancy Reagan once wrote that nothing could prepare you for living in the White House," the Obamas wrote in a joint statement on Sunday after news of Reagan's death in California.

"She was right, of course. But we had a head start, because we were fortunate to benefit from her proud example, and her warm and generous advice.

"Our former first lady redefined the role in her time here. Later, in her long goodbye with President Reagan, she became a voice on behalf of millions of families going through the depleting, aching reality of Alzheimer’s, and took on a new role, as advocate, on behalf of treatments that hold the potential and the promise to improve and save lives.

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"We offer our sincere condolences to their children, Patti, Ron, and Michael, and to their grandchildren. And we remain grateful for Nancy Reagan's life, thankful for her guidance, and prayerful that she and her beloved husband are together again."