* S&P 500, Nasdaq to add to string of quarterly gains
* BNY Mellon shares up as Trian discloses stake
* MannKind rallies on approval of inhaled insulin
* Indexes: Dow flat, S&P 500 up 0.1 pct, Nasdaq up 0.2 pct (Updates to morning trading, adds comment)
By Rodrigo Campos
NEW YORK, June 30 (Reuters) - U.S. stocks were little changed on Monday following mixed economic 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.
Following a 30 percent gain on the S&P 500 index last year, many investors had expected a halt to an equity bull market that is now into its fifth year.
Still, even as the U.S. Federal Reserve has gradually been easing back its economic stimulus, major indexes are up for the first half of the year and the S&P 500 has posted more than 20 record highs at the close. Bond yields, expected to have risen after a run-up late last year, have broadly fallen.
"The Fed and other global monetary forces have done their best to keep this market as liquid as possible, and that liquidity is restricting investors from finding a place other than stocks and have enabled risk takers to stay confident," said Rick Meckler, president of investment firm LibertyView Capital Management in Jersey City, New Jersey.
The S&P closed Friday just 0.1 percent below its record close set a week earlier.
On Monday, the market barely budged after data showed the pace of business activity in June in the U.S. Midwest dipped more than expected from a seven-month high, and contracts to buy previously owned U.S. homes hit an eight-month high in May.
For the second half of the year "we need follow-through in economic data or we're likely to see the same drift in stocks," said Meckler.
"Either stocks come off because earnings don't deliver or bond yields rise because growth is finally realized."
The Dow Jones industrial average rose 5.08 points, or 0.03 percent, to 16,856.92, the S&P 500 gained 2.47 points, or 0.13 percent, to 1,963.43 and the Nasdaq Composite added 9.63 points, or 0.22 percent, to 4,407.56.
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, some traders say, has been frustratingly placid.
Barring a market slide on Monday, the S&P and Nasdaq will close up for a sixth straight quarter. That is the longest streak of quarterly advances for the Nasdaq since 2000, while for the S&P, it is the best run of quarters since 1998. The Dow will close its fifth positive quarter of the last six.
Bank of New York Mellon shares rose 3.1 percent to $37.34 after Trian Fund management disclosed in an amended filing a stake of more than 9 million shares of the bank.
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 10.3 percent to $11.03.
Markets 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.
Editing by Bernadette Baum