What happened

Shares of Greenwich LifeSciences (GLSI -1.00%) were tanking 27.8% as of 3:34 p.m. EDT on Monday. The huge drop came after the company published a poster over the weekend at the American Association for Cancer Research (AACR) Annual Meeting showing five-year immune response data from a phase 2b study of immunotherapy GP2.

So what

You might think that Greenwich's phase 2 results were horrible after seeing the biotech stock plummet today. That wasn't the case, however. The company reported that patients with HER2-positive breast cancer receiving GP2 had a 100% disease-free survival rate over five years.

Test tube rack with a broken test tube spilling a green solution.

Image source: Getty Images.

So what's behind today's sell-off? Perhaps the best explanation is that we're seeing the old adage "buy the rumor, sell the news" at work. Greenwich's share price jumped more than 50% in the week leading up to the AACR meeting. 

Also, stocks with unusually low floats can have some wild and unexpected swings at times if big shareholders decide to buy or sell. Greenwich's float is only around 2.8 million shares.

Now what

Greenwich LifeSciences now plans to advance GP2 into a phase 3 clinical study. Based on the data from the phase 2 study, the company thinks that monitoring immune response will be a key part of its late-stage trial.