Key Moments
- Western Digital Corp. (NASDAQ:WDC) gained 1.7% after Nasdaq named it to the Nasdaq-100 Index during the annual reconstitution.
- At the same time, Nasdaq added six companies, including Seagate Technology Holdings (NASDAQ:STX) and Alnylam Pharmaceuticals (NASDAQ:ALNY), while removing six others such as Biogen (NASDAQ:BIIB) and lululemon athletica (NASDAQ:LULU).
- The updated Nasdaq-100 composition will take effect before the market opens on Monday, December 22, 2025.
Western Digital Rallies on Index Inclusion
Western Digital Corp. (NASDAQ:WDC) shares rose 1.7% on Monday after Nasdaq selected the company for inclusion in the Nasdaq-100 Index as part of its annual reshuffle.
Notably, Western Digital joins the index as one of six new constituents. The Nasdaq-100 tracks the performance of the 100 largest non-financial companies listed on the Nasdaq Stock Market.
New Additions and Market Reaction
Meanwhile, other newly added stocks also moved higher following the announcement. Seagate Technology Holdings (NASDAQ:STX) climbed 2%, while Alnylam Pharmaceuticals (NASDAQ:ALNY) advanced 0.9%.
In addition, Ferrovial SE (NASDAQ:FER) gained 0.7%, Insmed (NASDAQ:INSM) rose 1.2%, and Monolithic Power Systems (NASDAQ:MPWR) increased 1.3%, reflecting broader optimism around index inclusion.
| Company | Exchange Symbol | Action | Share Move |
|---|---|---|---|
| Western Digital Corp. | NASDAQ:WDC | Added | +1.7% |
| Seagate Technology Holdings | NASDAQ:STX | Added | +2% |
| Alnylam Pharmaceuticals | NASDAQ:ALNY | Added | +0.9% |
| Ferrovial SE | NASDAQ:FER | Added | +0.7% |
| Insmed | NASDAQ:INSM | Added | +1.2% |
| Monolithic Power Systems | NASDAQ:MPWR | Added | +1.3% |
| Biogen | NASDAQ:BIIB | Removed | Not specified |
| CDW Corporation | NASDAQ:CDW | Removed | Not specified |
| GlobalFoundries | NASDAQ:GFS | Removed | Not specified |
| lululemon athletica | NASDAQ:LULU | Removed | -1.1% |
| ON Semiconductor | NASDAQ:ON | Removed | Not specified |
| The Trade Desk | NASDAQ:TTD | Removed | Not specified |
Implementation Timeline and Index Mechanics
Nasdaq confirmed that the revised Nasdaq-100 membership will take effect before the market opens on Monday, December 22, 2025. The exchange conducts this reconstitution annually each December.
Additionally, Nasdaq aligns the process with the quarter’s quadruple witching expiration Friday. As part of the rebalance, six existing members will exit the index.
Following the announcement, shares of lululemon athletica (NASDAQ:LULU) fell 1.1%, highlighting the short-term impact of removal on stock performance.
Significance of the Nasdaq-100 for Investors
The Nasdaq-100 Index plays a central role for global investors. According to Nasdaq, more than 200 financial products track the index, representing over $600 billion in assets under management worldwide.
Most importantly, the Invesco QQQ Trust (NASDAQ:QQQ) stands as the largest of these products. The ETF aims to track the Nasdaq-100’s performance before expenses, making index changes closely watched by both institutional and retail investors.
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