* S&P 500, Nasdaq on six-day winning streak as of Tuesday
* Gilead Sciences rises in premarket after results
* Key earnings on tap; Apple, Facebook out after the close
* Futures down: Dow 5 pts, S&P 2.1 pts, Nasdaq 7 pts
By Ryan Vlastelica
NEW YORK, April 23 (Reuters) - U.S. stock index futures were flat on Wednesday as investors found few reasons to keep buying following six straight days of gains on both the S&P 500 and Nasdaq.
* Equities have recently been boosted by corporate earnings, with a number of prominent names topping expectations, though companies have largely been beating reduced forecasts. According to Thomson Reuters data, profits are seen rising 1.1 percent this quarter, down from the 6.5 percent growth rate estimated at the start of the year.
* Gilead Sciences Inc rose 3.8 percent to $75.60 in heavy premarket trading a day after the drugmaker reported a sharp increase in its quarterly profit, helped by sales of its hepatitis C drug. Gilead's results, like those of Netflix Inc , showed there was still room to grow in high-flying names that recently plunged on signs of excessive valuation.
* Shares of Boeing Co rose 2.3 percent to $130.50 after the planemaker reported its first-quarter results and raised its full-year outlook for core earnings.
* Procter & Gamble Co reported third-quarter sales that were slightly under expectations. The Dow component's chief executive said the company was operating "in a slow-growth, highly competitive environment."
* More key reports are expected Wednesday, with Apple Inc and Facebook Inc scheduled for release after the market closes.
* With 20 percent of the S&P 500 having reported results through Tuesday morning, 63 percent have topped earnings expectations, according to Thomson Reuters data, matching the long-term average. On the revenue side, 51 percent have exceeded forecasts, below the 61 percent long-term average.
* 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 dipped 5 points and Nasdaq 100 futures lost 7 points.
* Both the S&P 500 and Nasdaq closed out six straight days of gains on Tuesday. The S&P rose 3.5 percent over that period while the Nasdaq added 4 percent.
* In another positive sign for equities, the Dow Jones Transportation Average rose 0.6 percent on Tuesday to close at a record high, its first since April 2. However, trading volume was light, suggesting a lack of investor conviction behind the move.
* AT&T Inc late Tuesday raised its full-year revenue growth outlook, citing its acquisition of LEAP wireless in March and the popularity of a new handset pricing model that charges customers for devices separately from their wireless plans. Shares of the Dow component fell 1.4 percent to $35.78 in premarket trading.
* JetBlue Airways Corp pilots late Tuesday voted by a wide margin to join the Air Line Pilots Association union, raising concerns that the move would raise the airline's costs. Shares fell 0.9 percent to $8.67 in light premarket trading. (Editing by Chizu Nomiyama)