elizabeth neufeld was studying patients who had multiple inclusion bodies present in their cells. due to the large amount of inclusion bodies she named this condition i-cell disease. these inclusion bodies represented lysosomes that were filled with undigestable material. at first neufeld thought these patients must have a lack of lysosomal enzymes. . further study showed that all of the lysosomal enzymes were being made but they were being incorrectly targeted. instead of being sent to the lysosome, they were being secreted. furthermore, these mis-targeted enzymes were found to not be phosphorylated. therefore, neufeld suggested that i-cell disease was caused by a deficiency in the enzymes that add a specific mannose 6-phosphate tag onto lysosomal enzymes so they can be targeted to the lysosome.