What happened

Shares of cancer-treating biopharmaceutical company Celgene (NASDAQ:CELG) sold off on Thursday, closing down 8.6% after yet another high-profile shareholder of Bristol-Myers Squibb (NYSE:BMY) -- the even bigger Big Pharma company that's trying to buy Celgene -- announced that it opposes Bristol-Myers going through with the deal.

As The Wall Street Journal reported today, activist investor Starboard Value LP cited "risks inherent in this acquisition" combined with what it views as Bristol-Myers management's poor past performance as reasons for doubting the wisdom of this deal. The investor also reminded Bristol-Myers that Celgene is about to lose patent protection for its multiple-myeloma drug Revlimid, endangering future sales at Bristol-Myers' new prize.

Starboard's broadside came in the wake of similar objections from Bristol-Myers investors Dodge & Cox and Wellington Management Co.

Artist putting finishing touches on wall painting of large fish chasing smaller fish.

Will big fish Bristol-Myers eat little fish Celgene? Image source: Getty Images.

So what

Wellington, the largest holder among the three firms named, owns only about 7.7% of Bristol-Myers stock, according to data from S&P Global Market Intelligence. Combined with Dodge & Cox's 2.6% stake, that's still barely 10% of Bristol-Myers' shares now on record opposing the sale -- not enough to scuttle the deal.

As for Starboard, it's believed to currently own just a fraction of 1% of Bristol-Myers Squibb's shares. This being the case, investors selling off Celgene on fears that Bristol-Myers Squibb will suddenly decide to back out of the deal may be jumping the gun. Indeed, already today, two investment bankers -- Jefferies and William Blair -- went on record saying they think the deal will close on schedule.

Now what

That being said, there's still six weeks to go before Bristol-Myers shareholders are scheduled to vote on this deal. With opposition continuing to mount and plenty of time to organize more opposition before April 12, it's impossible to totally discount the risk that this deal could fall through.