The US Food and Drug Administration has halted Geron's stem cell trial, even before the first patient received treatment. In August, the Menlo Park, California–based company learned that its potentially first-in-class therapy for spinal cord injury, GRNOPC1, neural cells derived from human embryonic stem cells, was suspended following news that animals in a dose-escalation study developed microscopic cysts in regenerating tissue sites. This is not the first time the trial stalled: the agency had previously halted the spinal cord injury clinical study in May 2008 to consult experts for the best approach forward, and the company finally got the go-ahead in January this year (Nat. Biotechnol. 27, 213–214, 2009). Geron claims that cysts are fairly common in humans with spinal cord injuries. In this case, it was unclear whether the cysts were endogenous or related to the therapy, and they did not develop into teratomas, the company stated in a press release. A set of cell markers “linked with cyst formation across all animal studies in which cysts were found” has been identified by Geron as part of their efforts to assess lot variability. This is something that the FDA is concerned about with all biologics, says Greg Bonfiglio, of Proteus Ventures in Palo Alto, California. Bonfiglio points out that the company reacted responsibly, having identified the problem in a mouse model, called it to the FDA's attention and addressed it. “It's all part of the process of developing a cutting-edge technology,” he says. “The field will ultimately benefit as it gets a better read of the safety issues.”