The original design of the Prochymal phase II study called for a control arm to be enrolled, with patients receiving steroids alone that could have been used to more accurately assess Prochymal's efficacy. Response in the original study also called for a measurement at day 42. But in 2005, right before the phase II study was to start, Osiris altered the study design to eliminate the control group and shorten the efficacy endpoint to measure response at day 28. The changes were made so the study could be run more quickly, the company says. That might be true, but it also lowered the bar by which Prochymal could be deemed a success. This may also explain why the ongoing phase III trial is only enrolling GVHD patients who no longer respond to steroids (a sicker patient population than enrolled the phase II study.) Osiris is enrolling 240 patients in this phase III study, split between patients getting Prochymal injection and a placebo. If positive, the company plans to seek FDA approval. Enrollment in the trial is expected to finish up this summer, with data ready for release by the end of the year. Treatment of steroid-refractory GVHD is an unmet medical need, but one with small commercial potential of around $30 million per year. Investors are right to be excited by companies developing cutting-edge stem-cell therapies. But the path to innovative new cures and treatments for life-threatening diseases is not going to be smooth, as Tuesday's disappointing news from Osiris illustrates. And while the stem-cell headlines may look sensational, the details are what will ultimately count most.