As in Mucolipidosis II (I-cell disease), Mucolipidosis III results from genetic defects in GlcNAc phosphotransferase (N-acetylglucosamine-1-phosphotransferase).
In Mucolipidosis II (I-cell disease), a defect in the enzyme GlcNAc phosphotransferase causes the condition.  Mucolipidosis III is also caused by a mutation in this same enzyme.