The Nasdaq stock exchange will cut the weighting it gives to Apple on a key index at the same time that it boosts the weighting for tech titans including Microsoft and Cisco Systems.

In a major rebalancing of the tech-heavy Nasdaq 100 index, which will go into effect ahead of the market open on May 2, the exchange aims to better reflect the current market values of several significant companies. For example, Apple's weighting on the Nasdaq 100 is currently more than six times that of Microsoft, even though Apple's market value is only 46 percent larger, according to data from Bloomberg.

As a result, Apple will see the portion that its shares represent on the index drop to 12.33 percent from 20.49 percent, a decline of more than 8 percentage points and the largest individual change. Microsoft's weighting will increase to 8.32 percent from 3.41 percent, a gain of almost 5 percentage points and the largest increase.

The need to rebalance the index was driven partly by the huge gains in Apple's stock price over the past couple of years, says The Wall Street Journal (subscription required). Overall, the move will affect mutual funds, exchange-traded funds, and other instruments that mirror the performance of the Nasdaq 100 and is likely to result in a flurry of trading as money managers scramble to rejigger their holdings.

Apple, in particular, could be in store for some aggressive selling. Its shares fell more than 3 percent today in early morning trading just past 6:30 am EST, according to Bloomberg, though they've since climbed back up.

"This would probably affect international funds, given that Apple is such a big company globally," Pengana Capital money manager Tim Schroeders told Bloomberg. "Passive funds in particular will be forced to reweight into those stocks that have had increases in weightings, and in the short term we will see some selling pressure on Apple."

Other tech players, including Cisco, Google, Intel, and Oracle will see their weightings grow by several percentage points. Every company on the index will be affected by the rebalancing, though the weightings of most will go up or down by only a fraction of a percentage point. Overall, only 18 of the 100 companies tracked will see an increase in their weightings.

In a presentation on its Web site (PDF), the Nasdaq explained that it started looking into rebalancing the Nasdaq 100 index early last year after it found that the weightings assigned to certain stocks were no longer accurately reflecting their real market values following an initial rebalancing in 1998.

The new rebalancing will go into effect May 2 and be based on the shares in the Nasdaq-100 as of March 31.