A leading indicator for stocks is breaking down

One leading indicator for the equity market is breaking below a key support level, a move that could have negative implications for stocks.

The iShares high-yield corporate bond exchange-traded fund, the HYG, has fallen below its 200-day moving average, which it had held above for much of this year. This move concerns Matt Maley, equity strategist at Miller Tabak. Here's why.

• The high-yield market has begun selling off in the past week, Maley wrote in an email to CNBC, leading the HYG to decline. As the 200-day moving average has proved solid support for the ETF, "this raises some concerns in my mind," he wrote.

• However, it would have to fall below its August lows of 87.15 to "raise a yellow warning flag on this asset class," he added.

• While the drop below the moving average is not a major problem just yet, he said, he'll be watching closely to see if the market shows further signs of weakness in the weeks ahead given the ETF's record as a strong leading indicator for stocks.

• The HYG and the S&P 500 have been closely correlated since late 2015, and the ETF is barely positive on the year.

Bottom line: The high-yield market is beginning to flash a warning sign for the broader market.

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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.

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