U.S. stocks may open lower Tuesday, as investors mull over the rebalancing of the Nasdaq-100 index to diminish Apple's influence, a rate hike in China and another downgrade of Portugal's debt. Nasdaq OMX Group announced Tuesday it will rebalance its tech-heavy Nasdaq-100 index, reducing Apple's (AAPL, Fortune 500) weight by almost 40% - to 12.3% from 20.5%. The change takes effect on May 2.The change will lend more weight to Google (GOOG, Fortune 500), Intel (INTC, Fortune 500), Microsoft (MSFT, Fortune 500) and Oracle (ORCL, Fortune 500).Apple shares fell nearly 3% in premarket trading. Microsoft, Intel and Oracle each nudged about 1% higher.The People's Bank of China also surprised investors Tuesday by announcing a quarter percentage point hike in interest rates, as part of its continued efforts to gradually slow down the country's rapidly rising prices.China's benchmark one-year lending rate now stands at 6.31%.Economy: At 14:00 GMT the Institute for Supply Management will release its monthly gauge on the service-sector index for March. Economists are looking for the index to slip slightly to 59.5, compared with last month's reading of 59.7.Later in the day, the Federal Reserve will release minutes from its March 15 policy meeting.Companies: KB Home (KBH) shares fell nearly 6% after the homebuilder announced a quarterly loss of $114.5 million, or $1.49 a share. That's far deeper than the loss analysts were expecting.Shares of rival homebuilder Lennar Corp. (LEN) fell 0.5% in early trading after the announcement.