By Fred Fuld

Last Tuesday, Geron ( GERN) spiked by as much as 21% after forming an alliance with GE Healthcare, a unit of General Electric ( GE), to develop test products from embryonic stem cells.

A likely contributor to Geron's quick rise was that more than 16% of the stock's float was shorted, which could have caused a short squeeze.

A short squeeze occurs when short-sellers quickly buy in shares of a stock in order to cover their bearish positions, driving the price of the stock up sharply. The ratio for measuring short squeeze opportunities, called the short ratio, reflects the number of days it would take the short-sellers to cover their positions based on recent average daily volume.

With Geron's recent move in mind, we thought we'd take a closer look at the biotech sector for other potential short squeezes. Stockpickr has reviewed the heavily shorted biotech stocks with market caps of more than $250 million and created a portfolio of the top biotech short-squeeze opportunities.

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