Goldman: Buy Amazon, Facebook & other mo' stocks

Traders work on the floor of the New York Stock Exchange.
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Investors are starting to get concerned about the sustainability of the bull market when just a handful of stocks are posting sizable gains.

In fact, without, Alphabet, Microsoft, Facebook and General Electric, the market would be 2 percent lower for 2015, according to Goldman Sachs.

"The Goldman Sachs Breadth Index currently equals 1, one of the lowest readings in the 30-year series. Our index has experienced only 11 narrow breadth periods since 1985, including three during the late 1990s that share several characteristics with the current narrow breadth episode," Goldman's David Kostin wrote in the note to clients Monday.

But the firm is telling clients not to worry, just jump into those few stocks leading the bull.

That's because, when this so-called narrow breadth occurs, megacap stocks with high price momentum and strong balance sheets typically keep outperforming for four more months, according to Goldman.

Here are the 10 Goldman momentum stocks the firm recommends to clients in order to take advantage of the trend.