I-Cell disease also known as Mucolipidosis II (ML II) is an inherited lysosomal storage disorder. In I-cell disease, many lysosomal enzymes are missing because the enzymes are lacking a signal that is necessary for them to get inside the lysosome. Instead of getting into the lysosome and breaking down the molecules found there, the enzymes are found outside the lysosome. This leads to the build-up of molecules inside the lysosome.

I-Cell disease is closely related to Pseudo-Hurler Polydystrophy (Mucolipidosis III). Both disorders are “targeting “ defects. Pseudo-Hurler Polydystrophy disease presents with much less severe clinical findings. The enzyme that is responsible for putting the targeting signal on the lysosome enzymes implicated in I-Cell disease and Pseudo-Hurler Polydystrophy is N-acetylglucosamine-1-phototransferase.