June 22 (Reuters) - Wall Street's major indexes were muted at open on Monday, with investors taking a breather after returning from Friday's public holiday while awaiting further developments in U.S.-Iran negotiations.

The Dow Jones Industrial Average fell 9.5 points, or 0.02%, at the open to 51,555.19. The S&P 500 fell 0.1 points, or 0.00%, to 7,500.44​, while the Nasdaq Composite dropped 34.6 points, or 0.13%, to 26,483.31.