Nasdaq this morning sent a wave through the stock market with the announcement that that it will “rebalance” its closely followed and largely tech-oriented Nasdaq 100 Index to reduce the influence of Apple and boost the impact of Microsoft, Oracle, Google Cisco and other tech giants. The news is a big deal in part because of the number of funds that mirror the Nasdaq 100 in their holdings, causing them to adjust to the change by buying or selling.

Microsoft shares were up modestly this morning. Apple shares were down slightly.

Why the change? Nasdaq explains that the goal is to bring the weights of the Index Securities closer in line with their actual market capitalizations. Apple’s weight on the index is currently more than 20 percent. Explains the WSJ:

The rebalancing was driven in part by the seemingly unstoppable rise in Apple shares, which are up more than fourfold in the past two years. The tech company’s big weighting means that a change in fortune for the maker of iPhones, iPods and iPads has a huge impact on one of the most heavily traded indexes in the market. After the rebalancing, which takes effect before the market open on May 2, Apple will make up 12% of the Nasdaq-100.

The changes takes effect May 2. Check out this Nasdaq presentation for details: PDF, 25 pages.

More from Reuters and Bloomberg News. And here’s a slice of a Nasdaq chart showing the changes …

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