This strategy will allow access to genetic counselling and molecular diagnosis (positive, presymptomatic or prenatal diagnoses), based on a rational strategy from phenotype-genotype correlations and the information concerning the relative frequency of the genes. The detailed description, with the help of new evaluation tools and of the follow-up of the natural history of the disease according to the genotype, constitutes a crucial step in the design of therapeutical trials in these orphan disorders. Furthermore, the regular follow-up by specialized centers will allow better care of the patients.