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What: Shares of Juno Therapeutics, Inc. (NASDAQ:JUNO) a clinical-stage biopharmaceutical company developing cell-based cancer immunotherapies, fell 24% in July, according to data from S&P Global Market Intelligence. The FDA placed a clinical hold on the company's lead program, bringing all trials with JCAR015 to a standstill. The agency removed the hold much more quickly than it had to, but the stock hasn't been as responsive.

JUNO Chart

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So what: Juno's JCAR015 is a Celgene-partnered program that involves removing a patient's T-cells, and then adding a single gene in a laboratory setting that encourages recognition and destruction of cancerous cells in patients with acute lymphoblastic leukemia (ALL).

Although ALL is the most common form of cancer and leukemia in U.S. children, treatment options are somewhat limited. A majority of children with ALL are cured with standard chemotherapy, but the same approach cures only 20% to 30% of adults.

Juno's JCAR015 fired up oncologists with results from a phase 1 trial that enrolled 51 adult ALL patients with disease progression after standard therapy. During the study, a stunning 41 patients showed a complete response, and among the complete responders, 33 exhibited complete molecular remission. In other words, for about 66% of evaluable patients who had run out of options, JCAR015 treatment bashed their disease down to levels undetectable with incredibly sensitive diagnostic equipment.

The phase 2 "Rocket" trial with JCAR015 has been designed to support accelerated FDA approval for these patients as soon as next year, but two patients' deaths caused the FDA to halt the trial, nearly derailing the program.

Juno isolated the issue to the addition of a specific chemo drug used to precondition patients before infusion with JCAR015 cells. The FDA could have taken much longer to respond to the change in Juno's procedure, but it lifted the hold just five days after placing it.

Now what: The agency's willingness to expedite its review of Juno's solution to the hold in mere days highlights just how important it views these therapies. Juno is hardly the only company modifying T-cells to seek and destroy various cancers, and its peers' stock prices enjoyed a nice bump.

The high tide, however, failed to lift Juno's boat. Perhaps investors fear removal of the problematic chemotherapy from the preconditioning regimen might not solve the problem. Juno estimates the annual U.S. ALL market size at about $250 million. Success in the resumed phase 2 Rocket trial could send the stock soaring, but another patient death could also lead to its collapse.

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