The iShares Nasdaq Biotechnology ETF (NasdaqGM: IBB) lost another 1.1% yesterday. On its own, that does not sound like much, but the largest biotech exchange traded fund has lost ground in four of the past five sessions, a span in which IBB is off almost 4.7%.

Some major biotech ETFs have been worse than IBB over that period, stoking speculation that a group of stocks and ETFs that have been major contributors to the upside in U.S. stocks for several years is in danger of wilting.

“Aside from the steep slope of the trend we can also see a bearish condition in momentum. While prices made higher highs, momentum indicators such as the relative strength index (RSI) made a lower high in a condition called a bearish divergence. It means that the power behind the rally is waning and a breakdown is now a good possibility,” reports Michael Kahn for Barron’s.

“According to data from CNBC, 81 percent of stocks in the IBB with a market cap of more than $500 million are down more than 10 percent from 52-week highs, technically placing them in correction territory,” reports Stephanie Yang for CNBC.

The $9.49 billion IBB is home to 145 stocks, but the fund’s top 10 holdings combine for nearly 60% of its weight. IBB’s top four holdings combine for nearly a third of the ETF’s weight and in the case of IBB, which is a cap-weighted, downturns for the largest biotech stocks can mean trouble for the fund. IBB’s top four holdings are Celgene (NasdaqGS: CELG), Amgen (NasdaqGS: AMGN), Gilead Sciences (NasdaqGS: GILD) and Regeneron Pharmaceuticals (NasdaqGS: REGN).

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