Amicus Therapeutics ( FOLD) presented some phase II data on its Fabry disease drug Amigal on Wednesday. Amigal appears to be an active drug that can reduce the toxic buildup of a protein called GL-3 in the kidneys of patients. It's the inability to remove GL-3 from cells in the body (because of a genetic malfunction) that causes the severe physical problems in Fabry patients. However, investors' reaction to what seemed like positive news was anything but. Amicus shares were shellacked Thursday, falling $5.59, or 37%, to $9.54. Why the incongruence between the Amigal data and the stock price? The answer is in the details of the Amigal data. Amicus got its best results by measuring reductions in GL-3 levels in the urine of Fabry patients. The efficacy was less robust, however, when GL-3 levels were measured via biopsies of kidney cells.