CRANBURY, NJ ( TheStreet) -- Amicus Therapeutics ( FOLD) presented an alternative theory Friday to explain the failure of its Fabry disease drug Amigal in a late-stage clinical trial. Not surprisingly, investors reacted with skepticism and Amicus shares sank 25% to $2.89. It's easy to dismiss Amicus' analysis for what it looks like on the surface: Just another example of a drug company resorting to retrospective data mining in order to stave off the painful reality of a negative clinical trial. But this time, what Amicus is saying about Amigal might be right: The drug works against Fabry disease. The challenge -- more difficult than the company anticipated -- is designing a clinical trial to prove it. Here's what happened Friday: Amicus dissected the Amigal phase III study and found slightly more than half of the Fabry patients had relatively mild disease at the start of the trial, way more than the company intended to enroll. Mild Fabry disease is harder to measure so in these patients, the study was unable to detect a significant difference in kidney response between Amigal and placebo. However, in Fabry patients with severe disease at baseline and therefore more measurable, the difference in kidney response detected between Amigal and placebo was greater. Had the study enrolled more patients with severe disease, as originally planned, the difference in response between Amigal and placebo might have been statistically significant, the company said. Now, to be clear, Amicus conducted this six-month analysis on a post-hoc basis, meaning separating patients by disease severity was not part of the original statistical analysis plan. However, Amicus is speaking to FDA about amending the study design before the next scheduled look at the Amigal data at 12 months. Amicus expects 12-month data to be ready late in the second quarter. Investors have largely written off Amigal today -- and it's hard to blame them -- but skepticism about the drug could turn into optimism if the 12-month data are strong. If that happens, Amicus shares could rebound and more. To understand why Amigal chances shouldn't be discarded quite yet, it's important to go back over some background on Fabry disease and review the design and initial results from the phase III study.