Nasdaq OMX Group elected to conduct a special rebalancing of the popular Nasdaq-100 Index after a rapid rise in some of the benchmark’s stocks, an executive at the exchange said in an interview Tuesday.

Nasdaq said the index rebalance would sharply reduce Apple’s weighting. The stock currently accounts for more than 20% of the index, resulting in concentration at Nasdaq-100-tracking exchange traded funds (ETFs) such as PowerShares QQQ (NasdaqGM: QQQ).

John Jacobs, executive vice president at Nasdaq’s global index group, said the company looked at rebalancing the Nasdaq-100 early last year because the stock weightings had drifted away from actual market capitalization.

However, he said the firm didn’t want to disrupt the re-ranking of Nasdaq-100 stocks, which is announced in December.

“So we wanted to do any change in the first part of the year,” Jacobs explained.

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