Mission Bay's biggest landlord is about to make a big move.

Alexandria Real Estate is under contract to buy Block 26/27, better known as 1455 and 1515 Third streets, sources tell us. We are hearing the price could be about $125M, and that includes existing infrastructure costs (so the net number for the land is lower). Above, Alexandria CEO Joel Marcus at Rock Health's Mission Bay opening earlier this year. He knew planting a flag years ago in Mission Bay—then a desolate land—was a gamble. Now Alexandria calls Bayer, Celgene, and Kaiser tenants there.

The site is just north of where the Warriors stadium is going to land. Last month at Bisnow's Future of the Waterfront event, SF Office of Community Investment and Infrastructure exec director Tiffany Bohee said the promise of Mission Bay is just starting to be fulfilled, having gotten its entitlements back in 1998. Because plans were already approved it allows development to move forward in spite of Prop B.