Mucolipidosis type 4 is a metabolic condition that affects the body's ability to process certain carbohydrates and fats. As a result, these materials accumulate in cells leading to the various signs and symptoms of the condition. Most people with mucolipidosis type 4 develop severe psychomotor (mental and motor skills) delay by the end of the first year of life and visual impairment that worsens over time. Other common features of the condition include limited or absent speech; intellectual disability; hypotonia that gradually progresses to spasticity; problems controlling hand movements; impaired production of stomach acids; and iron deficiency. Approximately 5% of affected people have a mild form of the condition (known as atypical mucolipidosis type 4) which is associated with milder psychomotor delay and less severe eye abnormalities. Mucolipidosis type 4 is caused by changes (mutations) in the MCOLN1 gene and is inherited in an autosomal recessive manner. Treatment is based on the signs and symptoms present in each person.