Genzyme Corp. has leased 251,000 square feet for a new headquarters at 50 Binney St. in Cambridge and will relocate from 500 Kendall Square in 2018.

The new headquarters will accommodate 1,300 employees, up from 900 in the current headquarters. Some employees from other divisions of Paris-based Sanofi, Genzyme’s parent company, will move to the new building as well, Genzyme spokesman Bo Piela said. Others will remain in existing offices in Cambridge.

50 Binney is one of three approved build-to-suit sites totaling nearly 1 million square feet at Alexandria Center at Kendall Square, an 11-acre property owned by Alexandria Real Estate Equities.

Vacancy rates for office and lab space in Kendall Square are nearing historic lows as biotech companies continue to cluster in the industry hub.

The vacancy rate for East Cambridge lab space was 3.3 percent in the first quarter of 2015, according to a report issued last week by commercial real estate brokerage DTZ, while class A office space had a 7.3 percent vacancy rate.