Mucopolysaccharidosis type I (MPS1), also known as Hurler syndrome, is a rare inherited disorder caused by deficiency of the alpha-L-iduronidase (IDUA) enzyme. IDUA is necessary for the breakdown of large sugar molecules. Accumulation of these sugar molecules prevents cells, tissues, and organs from functioning correctly. There are two forms of Hurler syndrome: severe MPS1 and attenuated MPS1. Children affected by severe MPS1 appear normal at birth but begin to show signs and symptoms within the first year of life. They may have a large head, buildup of fluid in the brain, heart disease, enlarged liver and spleen, and large tongue. Children may also develop distinctive facial features. All children develop progressive skeletal dysplasia, as well as progressive intellectual disability. Some children will have hearing and vision loss. Death usually occurs in the first decade of life. Individuals affected by the attenuated MPS1 usually have milder progression of the disease with the possibility of longer life expectancy. Currently there is no cure for Hurler syndrome, although bone marrow transplant and enzyme replacement therapy can help improve some but not all symptoms if treatment begins early enough in life. Hurler syndrome is caused by pathogenic variants in the IDUA gene.