Shares of Juno Therapeutics plunged Friday after halting a midstage study on a potential leukemia treatment following two patient deaths.

The stock fell more than 26 percent in pre-market trading about two hours before the market open.

The biotechnology company is developing a treatment for a type of leukemia. It said Thursday that the study has been put on hold after it added the chemotherapy drug fludarabine to the regimen.

It has proposed continuing the study without fludarbine.

Midstage studies focus on a potential drug’s effectiveness and are a key step in the development and eventual approval process.

Shares of Juno Therapeutics Inc. dropped $10.61 to $30.21 in premarket trading about two hours before the market open.