[3] Most patients with ML IV show psychomotor retardation (i.e., delayed development of movement and coordination), corneal opacity, retinal degeneration and other ophthalmological abnormalities.

Other symptoms include agenesis of the corpus callosum, iron deficiency resulting from an absence of acid secretion in the stomach, achlorhydria.

An important property of mucolipin1 is that decreasing pH (acidification) results in deactivation of the protein, likely through an assembly defect.

[6] Mutations that alter only the C-terminal of the protein also result in a mild phenotype of the disorder, usually sparing the brain.

[7] ML IV causes affected cells to accumulate auto-fluorescent vacuoles considered to be aberrant lysosomes.

[citation needed] It is not yet clear why these abnormalities will cause incomplete development of the brain, achlorhydria, and failure in the maintenance of retinal tissue.