Nasdaq 100 rally can be credited to just two stocks

While the S&P 500 and the Dow have had a mild 2015, rising 1.1 percent and falling 0.5 percent, respectively, another widely followed large-cap index is having a great year: The Nasdaq 100.

The technology-heavy set of stocks has risen by 10 percent this year.

But that doesn't mean every tech investor has done well. In fact, the outperformance can be squarely pegged on just two names: Amazon and Alphabet (formerly known as Google).

Those two companies (which technically comprise three stock tickers, given that two different flavors of Alphabet shares are in the index) have added 478 points to the Nasdaq 100 in 2015, according to FactSet. The index itself has only risen by 419 points this year.

That means that incredibly, if not for Amazon and Alphabet, the outperforming Nasdaq 100 would actually be down on the year.

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Further exclude third- and fourth-largest contributors Facebook and Netflix, and we're looking at an index that's actually down 5 percent in 2015.

Of course, some stocks always perform better than others. But the narrow leadership of the Nasdaq 100 has become a major topic of conversation on Wall Street.

"The money continues to crowd into the same names. The gap between the FANG stocks and the SPX remains wide," Michael Block, chief strategiest of Rhino Trading Partners, wrote in a Thursday note, referring to an acronym for Facebook, Amazon, Netflix and Google and to the S&P 500 index.

"This will eventually end in tears," Block warned.

Larry McDonald, head of U.S. strategy at Societe Generale, believes that such "crowding" is reflective of general fears about the market on the part of asset managers.

"What you're really seeing is fear," he told CNBC. "Let's say that you have to be invested in stocks, or perhaps in growth stocks specifically. Well, if you're nervous about the market, you're going to pick the biggest, most liquid names possible — your Facebooks, your Googles, your Amazons."

For that reason, McDonald sees the stunning outperformance of a few choice names as an ironic reflection of bearish sentiment that will eventually take the overall market lower.

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Michael Santoli

Michael Santoli joined CNBC in October 2015 as a Senior Markets Commentator, based at the network's Global Headquarters in Englewood Cliffs, N.J.  Santoli brings his extensive markets expertise to CNBC's Business Day programming, with a regular appearance on CNBC's “Closing Bell (M-F, 3PM-5PM ET).   In addition, he contributes to CNBCand CNBC PRO, writing regular articles and creating original digital videos.

Previously, Santoli was a Senior Columnist at Yahoo Finance, where he wrote analysis and commentary on the stock market, corporate news and the economy. He also appeared on Yahoo Finance video programs, where he offered insights on the most important business stories of the day, and was a regular contributor to CNBC and other networks.

Follow Michael Santoli on Twitter @michaelsantoli

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