"I understand it's done," says one downtown broker familiar with the deal. The source estimates the lease at 90,000 sf, maintaining that Viant will be taking the last major piece of 400,000 sf left by Blue Cross and Blue Shield of Massachusetts. The health insurer, which owned half the building before selling its share to Equity Office Properties, is relocating across town to new headquarters in the Fenway District.

Calls to Viant Corp. were not returned by press deadline, while an Equity spokeswoman declined comment on the matter. Nonetheless, sources say the deal has been cemented, with one broker placing the rental rate at more than $60 per sf. Since buying the property in 1998 for $222.5 million, Equity has undertaken an extensive overhaul of the 32-story, 1.06 million-sf tower. Most of the Blue Cross space was snapped up earlier this summer by Fidelity Investments, which leased approximately 200,000 sf there.

Viant is currently located in a class B building on South Street, a few blocks away from 100 Summer St. The company saw its stock plummet by 42% last week after reporting its quarterly numbers would be below expectations.