Leighton Meester and Vera Wang joined forces today for a great cause in the Big Apple. The actress paired Brian Atwood pumps with a floral skirt to help the designer light the Empire State Building red and white in honor of tonight's annual DKMS gala. DKMS is the world's largest bone marrow donation center, and its star-studded event helps raise funds for leukemia prevention. This year, Rihanna will be recognized for her work on behalf of the cause, joining past honorees like Cindy Crawford. The pop star became an advocate a few years ago when a young fan suffering from the disease, who has since passed away, reached out to her and sparked her to get involved with DKMS. Leighton is taking the opportunity to give back during her hiatus from Gossip Girl and will toast Rihanna alongside celebs like Pharrell Williams and Eva Mendes at this evening's charitable soiree.