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 and visual impairment. 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 in the MCOLN1 gene and is inherited in an autosomal recessive manner.