The leader of Alexandria Real Estate Equities' operations in Greater Boston abruptly resigned last month, leaving his longtime post at the nation’s largest owner of life sciences real estate.

Former Alexandria co-President and Regional Market Director of Greater Boston Thomas Andrews resigned Dec. 30 for personal reasons, according to a Securities and Exchange Commission filing posted Monday.

Andrews, who had been at Alexandria since 1999, headed his company’s major life sciences deals in and around Boston as it led the charge in building up the nation's premier life sciences market. Hunter Kass, Alexandria’s strategic market director in Boston, was promoted to executive vice president to fill Andrews’ position.

Alexandria Executive Chairman and founder Joel Marcus declined to comment Monday. Kass didn’t respond to a message requesting comment.

It is unclear what prompted Andrews to step down, but until recently he had been active in the area's real estate community. He was named president of NAIOP Massachusetts for 2021 a few weeks before he stepped down from his post at Alexandria. A NAIOP representative confirmed Andrews is going to stay leading the trade association's local chapter this year.

Alexandria hired Andrews in 1999 after he served 11 years as executive director of the Massachusetts Biotechnology Research Park in Worcester. Alexandria credited Andrews with steering regional growth at its Kendall Square life sciences campuses. Alexandria currently owns more than twice as much space as the next-most-active developer in Boston with 7.4M SF, according to Newmark.

In 2019, Alexandria partnered in joint ventures spending a combined $333M for Seaport developments, and Alexandria later boosted its own presence last November with a $168M parcel purchase behind Boston’s Convention And Exhibition Center. Alexandria also purchased Athenahealth’s 11-building campus in Watertown for $526M in December 2019.

Kass, who joined Alexandria in 2018, previously worked for MIT’s endowment on its real estate team, leading the university's leasing efforts topping 1M SF and capital market transactions exceeding $2B over a six-year period.

UPDATE, JAN. 5, 9 A.M. ET: The Massachusetts chapter of NAIOP confirmed Andrews will remain chapter president in 2021.