The SPDR S&P Biotech ETF XBI is trading below an important resistance level, but the iShares Nasdaq Biotechnology ETF IBB has broken out.

Resistance is a large group of sellers who are all trying to sell their shares at the same price. When stocks or ETFs reach resistance levels, rallies pause or end.

When a stock trades above a level that had been resistance, it means the sellers who created the resistance are gone. With this supply taken off of the market, buyers will need to pay higher prices. This results in a rally and it's called a breakout.

XBI is trading below important resistance at the $140 level.

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But the chart of XBI looks better. It has broken out above the resistance at the $158 level.

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