The clinical presentation is highly variable from one patient to another regarding age at onset, severity, progression rate and life expectancy. Most patients are affected with the classical phenotype characterized by short trunk dwarfism with short neck and adult height < 1 m. Atypical phenotypes with less severe extension of skeletal manifestations, adult height > 1m, and less frequent complications in other organs have been progressively recognized. Clinical management differs depending on the clinical presentation of the patients but natural history of the disease is largely unknown in atypical phenotypes. Precise and exhaustive follow-up data are needed in such patients to increase our knowledge of this natural history and to define the best criteria to evaluate ERT efficiency.