SAN FRANCISCO—Transbay Tower, slated to be the tallest building West of the Mississippi upon completion (BASE jumpers are salivating), is about to get the first dent in its 1.4M SF of available office space. Sources told us this week that Salesforce is close to signing a 300k SF deal there.

Hines and Boston Properties teamed up last year to co-develop the tower, which is already underway, at First and Mission. It's right across the street from Salesforce's new 450k SF home at 350 Mission, which is being developed by Kilroy. (This means you won't be able to throw a stone on Mission without hitting a Salesforce employee. So be careful.) Hines senior managing director Paul Paradis, above, tells us there's no deals signed at the tower yet and couldn't comment on potential transactions.

The César Pelli-designed Transbay Tower will feed off of the future Transbay Transit Center, branded as the Grand Central Terminal of the West Coast. A Salesforce spokesman says the company doesn't comment on rumors, and its broker didn't respond to a request for comment. Salesforce has said it wants to occupy 1.8M SF in the city by 2016.