Apple is expanding and extending its lease for its flagship “cube” store at 767 Fifth Avenue.

The electronics retailer is expanding to more than 77,000 square feet at the General Motors building, Boston Properties president Doug Linde announced on the company’s earnings call Wednesday.

“They will grow from 32,000 square feet to more than 77,000 square feet. And while they are undertaking their expansion, they are using the former FAO Schwarz space on a temporary basis, Linde said.

Linde referred to Apple as the “mystery tenant” he’s mentioned on previous calls, but didn’t offer much in the way of further details.

Linde said Apple will turn former FAO space over to Under Armour to complete a full build-out.