U.S. stock index futures pointed to a slightly lower open on Thursday, the final day of the quarter, as jobless claims fell less than expected, but investors were optimistic about Friday's monthly jobs data.

Claims for unemployment benefits declined by 6,000 to 388,000 last week, the Labor Department said, in a sign of improvement in the labor market. Analysts polled by Reuters had forecast the level at 380,000.

The data followed an ADP report showing gains in private-sector jobs and precedes Friday's key non-farm payroll report for March from the government.

We'd like to see claims smaller than this, but this won't be earth-shattering for markets, said Michael Mullaney, a portfolio manager who helps manage $9.5 billion at Fiduciary Trust Co in Boston. ADP was relatively strong, so that number and this number don't rock the boat for tomorrow's payroll.

The week has been marked by some of the year's lowest volumes as traders opt to ride the quarter's gainers amid global risks. The S&P 500 is up 5.6 percent in the quarter, based on Wednesday's close.

David Sokol, the man widely seen as the leading successor to Warren Buffett to head up Berkshire Hathaway , has resigned after buying shares in chemical company Lubrizol Corp before pushing Buffett to acquire it.

In an interview on CNBC, Sokol said he did nothing wrong in buying the shares.

Berkshire's Class B shares fell 1.7 percent to $84 in premarket trading.

S&P 500 futures fell 2.1 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 slid 3 points and Nasdaq 100 futures slid 1.75 points.

February factory orders data will be released at 10 a.m. (1400 GMT) after the market opens. Factory orders are seen rising by a smaller amount than in the previous month.

Chicago PMI data for March, which covers Midwest business activity, will also be released.

Microsoft Corp stepped up its rivalry with Google Inc on Thursday by filing a formal complaint with the European Commission, claiming Google systematically thwarts Internet search competition.

The Macau unit of Las Vegas Sands Corp said it is being investigated by the Hong Kong Securities and Futures Commission for alleged regulatory violations. Shares fell 3.9 percent to $41.77 before the bell.

U.S. stocks rose for a second session on Wednesday with activity dominated by some of the strongest-performing groups in the first quarter, including energy and small-cap stocks.

(Editing by Kenneth Barry)