Mucolipidosis type IV (MLIV) is a neurodegenerative, recessive, lysosomal storage disorder characterized by psychomotor retardation and visual impairment due to various ophthalmologic abnormalities. MLIV is found in relatively high frequency in the Ashkenazi Jewish population. The disease is caused by mutations in the gene MCOLN1, which encodes the protein mucolipin 1 (MLN1), a member of the mucolipins family. MLN1 is a non-specific cation channel, and its putative structure attributes it to the TRP superfamily; thus, the gene is also referred as TRPML1. Over 16 MLIV-causing mutations, including two founder mutations in the Ashkenazi population, have been identified hitherto. Atypical increased lysosomal storage in MLIV is present in the cells of all patients. This accumulation is caused by an abnormal endocytosis process of the membrane components to late endosomes to the lysosomes, resulting in an apparent block in the traffic process in pre-lysosomal vacuoles with intraluminal pH of >5.0. MLN1 was localized in cultured cells to late endosomes and lysosomes. The exact function of this cation channel in the late stages of lysosomal maintenance is currently under study.