U.S. stock index futures cut losses on Tuesday after Berkshire Hathaway Inc said it would acquire railroad Burlington Northern Sante Fe Corp for $100 per share in cash and stock.

* S&P 500 futures fell 6.90 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures lost 54 points, and Nasdaq 100 futures were off 6.25 points.

(Reporting by Leah Schnurr; editing by Jeffrey Benkoe)