Short sellers have many reasons to bet against a stock, some of which are hedging or part of a larger theme. Other reasons may be a bet against a company or sector, or in the case of biotech, it could be a bet against a coming drug study result or FDA review period. Either way, short sellers are often active in the biotech sector.

24/7 Wall St. has reviewed the short interest reports and short selling trends among the five largest biotech stocks, now that the new short interest data are out from the May 30 settlement date. This has been compared to the May 15 settlement date or prior period for more color.

We have also decided to look at the short interest in MannKind Corp. (NASDAQ: MNKD), as its stock keeps rising ahead of the crucial FDA review of its inhalable insulin.

Alexion Pharmaceuticals Inc. (NASDAQ: ALXN) saw its short interest fall to 3,793,857 shares as of May 30, which was 3.88 days to cover. The short interest on May 15 was 4.109 million shares, but we would point out that this was the highest short interest over the past year. This may be a drop for the end of May, but this drop still represented the highest short interest going back to last July.

Amgen Inc. (NASDAQ: AMGN) saw its short interest rise to 8,529,579 shares at the end of May from 8,158,402 shares short in mid-May. This is still just under three days to cover here. And even with a 4.5% gain in the short interest, this was the second lowest reading since last September. Amgen may represent the only major biotech stock that is not seeing elevated short seller interest.

Biogen Idec Inc. (NASDAQ: BIIB) may look like a small number of shares short at 3,230,692 as of May 30, but this is deceiving because of that $315 share price. This also is 2.8 days to cover. The gain is up only 2% from the 3,167,801 shares short in mid-May. Still, what stands out here is that this was the highest short interest reading in a year.

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Celgene Corp. (NASDAQ: CELG) saw its end of May short interest fall to 6,290,425 shares, down 5.2% from the 6,637,466 shares short in mid-May. This was actually a slightly higher days-to-cover ratio of 2.65 (from 2.56). The short interest remains elevated here, up about 40% or so from the end of 2013.

Gilead Sciences Inc. (NASDAQ: GILD) saw its short interest fall to 94,135,575 shares at the end of May, down 4.2% from the 98,243,605 shares short in mid-May. Still, the days-to-cover ratio jumped to 9.11 days from 7.79 days. This was the first drop that you could really notice since the end of last summer, but this is still the third highest reading and mid-May was the highest reading of the last year. Gilead’s short interest remains a hot-bed as its hep-C vaccine has been under fire over its cost.

MannKind Corp. (NASDAQ: MNKD) may not be among the largest of the biotechs by any stretch with its $4 billion market cap. Still, it remains one of the biggest battleground stocks for bulls against short sellers. This is due to its inhalable insulin drug review at the FDA, a process that has been up for grabs for years — and not without controversy. MannKind’s short interest ticked up to 68,246,601 shares at the end of May from 68,035,239 shares short in mid-May. This was just shy of the peak in the short interest of the past year, and the short interest is up 50% from last December.

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