* S&P 500, Nasdaq to add to string of quarterly gains
* Chicago PMI, pending home sales data due
* MannKind rallies on approval of inhaled insulin
* Futures off: Dow 24 pts, S&P 2 pts, Nasdaq 1 pt
NEW YORK, June 30 (Reuters) - U.S. stock index futures were slightly lower on Monday ahead of business activity and home sales data, with the S&P 500 and Nasdaq Composite indexes set to close a sixth straight quarter of gains - a streak not seen in more than 14 years.
* The Chicago Purchasing Manager Index is due to be released at 9:45 a.m. EDT (1345 GMT) and the National Association of Realtors will publish its pending home sales index at 10:00 a.m. (1400 GMT).
* With an abbreviated week due to the U.S. Independence Day holiday on Friday and a ream of data due Wednesday and Thursday, there may be increased volatility in a market that has been frustratingly placid for some time.
* S&P 500 e-mini futures were down 2 points and fair value - a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract - indicated a slightly lower open. Dow Jones industrial average e-mini futures fell 24 points and Nasdaq 100 e-mini futures lost 1 point.
* Barring a market slide on Monday, the S&P and Nasdaq will close up for a sixth straight quarter. That's the longest streak of quarterly advances for the Nasdaq since 2000, while for the S&P, it's the best run of quarters since 1998. The Dow will close its fifth higher quarter of the last six.
* U.S. regulators on Friday approved MannKind Corp's inhaled insulin, saying the device offered a new treatment option for patients with diabetes. MannKind shares jumped 12 percent in premarket trading.
* Markets will continue to keep an eye on the Middle East as Iraqi troops battle for Tikrit after the leader of an al Qaeda splinter group was declared caliph of a new Islamic state in lands seized this month across a swathe of Iraq and Syria.
Reporting by Rodrigo Campos; Editing by Bernadette Baum