Donald Kohn, the 67-year-old vice chairman of the Federal Reserve Board, will step down when his term ends this coming June. Kohn has played a significant role behind the scenes, serving as an adviser to both Alan Greenspan and Ben Bernanke. His retirement will give President Obama an empty seat to fill on the seven-member board. Obama may choose someone from outside the Fed or could appoint Daniel Tarullo, a recent Fed appointee and former Georgetown Law professor, to the spot.