Symptoms are typically seen in the first few weeks of life and include impaired motor and mental ability and difficulty with swallowing. Other symptoms may include arthritis, swollen lymph nodes and joints, hoarseness, nodules under the skin (and sometimes in the lungs and other parts of the body), chronic shortening of muscles or tendons around joints, and vomiting. Affected persons may require the insertion of a breathing tube. In severe cases, the liver and spleen are enlarged. The disease occurs when both parents carry and pass on the defective gene that regulates the protein sphingomyelin. Children born to these parents have a 25 percent chance of inheriting the disorder and a 50 percent chance of carrying the faulty gene. The disorder affects both males and females.