WASHINGTON (CBSDC/AP) — Martin Sullivan, the Smithsonian’s National Portrait Gallery director, is stepping down at the end of May due to health reasons.

The museum announced Monday that Sullivan will take on the new role of senior scholar at the Smithsonian beginning June 4. Sullivan has led the National Portrait Gallery since 2008. Previously he led Historic St. Mary’s City Commission in Maryland.

Sullivan has overseen major exhibits including the long-term installation, “The Struggle for Justice,” and the award-winning and controversial exhibit “Hide/Seek: Difference and Desire in American Portraiture,” which explored the impact of sexual orientation on art history. The museum has showcased diverse portraiture during his tenure, including photographs of leading African Americans and Asian Americans.

Curator Wendy Wick Reaves will serve as acting director during a search for a new leader.

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