Brooklyn's biggest developer, Bruce Ratner, is preparing to step down as chief executive of Forest City Ratner, the New York City subsidiary of his family's Cleveland-based Forest City Enterprises. Taking his place will be MaryAnne Gilmartin, who became executive vice president in 2007 . The switch will elevate her to Mr. Ratner's clear No. 2, as he continues on in his role as chairman. It was not entirely clear why Mr. Ratner was stepping down or when exactly he would be ceding his CEO responsibilities at the company, but according to one source, his departure will be "sooner rather than later, likely the next few months." Added another, "I wouldn't expect him to disappear. He'll still be very much involved at Forest City." A spokesman for the company declined to comment. The news comes after a banner 2012 for Mr. Ratner. After a decade-long struggle to realize the biggest project of what has been a very big career in New York City real estate, Mr. Ratner finally opened the Barclays Center, the jewel of his 16-acre Atlantic Yards project in the heart of Brooklyn. That was followed last month by the groundbreaking for the first of what may ultimately be over a dozen apartment towers on the site. All will be built using modular construction, a process that has fascinated Mr. Ratner for some time and stands to revolutionize the way housing gets built in the city. Even the once-lowly Nets, the anchor tenant in the Barclays Center, in which Mr. Ratner remains a minority owner, have racked up a winning record this season.