A drugmaker's shares are crashing after two patients in a cancer trial died last week

Shares of Seattle-based biotech Juno Therapeutics' stock crashed Thursday, after news that its leading drug candidate led to two deaths last week.

The stock was down 30% as of 4:36 p.m. in New York.

Juno, which has been developing a treatment that uses genetically engineered cells to go after a form of adult leukemia, said that the phase 2 trial had been put on clinical hold by the FDA.

The type of treatment is known as CAR-T, or chimeric antigen receptor T cells. This kind of treatment, in Juno's case, uses a patient's own T-cells, a type of blood cell that's involved our immune system response.

The two deaths happened after Juno added fluradibine, a chemotherapy, to the treatment patients received before going through the gene-engineering process.

"This is a humbling experience," Juno CEO Hans Bishop told Forbes. "No doubt it is difficult for the physicians who are looking after these patients and their families. Clearly these therapies are potent, that's why they offer the potential for cures. We're still learning to use them in the safest, most efficacious way."

*An earlier version of this post misstated when the deaths occurred. It was last week, not this week.