Mastocytosis is a heterogeneous D-Pantothenic acid sodium disease characterized by mast cell accumulation in various organs. Some cases are complicated with organ insufficiency and/or symptoms due either to mast cell infiltration or mediators release. Tissues that are commonly involved are skin, bone marrow, gastrointestinal tract, liver and skeletal systems. The majority of mastocytosis cases occur in children, mostly as isolated cutaneous forms while less than 20% are complicated by a systemic dissemination. Most of the cases resolve by puberty. In contrast, patients with mastocytosis starting at the adult��s age present more often persistent systemic involvement. Stem cell factor is the major growth factor for mast cell survival and differentiation and interacts with its cognate receptor KIT, a tyrosine kinase encoded by the c-kit gene. C-kit protooncogene activation results in mastocytes accumulation and abnormal migration and activation in various tissues. In neoplastic mast cell lines, valine or tyrosine substitution for an aspartate in c-kit codon 816 results in constitutive phosphorylation and activation of KIT. In early reports, Asp816Val was found in peripheral blood of 27% of 55 adult patients with mastocytosis mainly of systemic forms or associated with clonal haematological disorders. Currently, it is well admitted that this mutation was also reported in skin and bone marrow in the vast majority of adult patients with systemic or cutaneous mastocytosis. In contrast, mutations in the c-kit gene are considered as rare in children. However, a recent study reports presence of 816 mutations in 11 out of 13 adults with the pediatric onset of cutaneous mastocytosis and in all children with systemic mastocytosis. Taken together these findings may explain differences in clinical presentation and outcome according to age of onset, and particularly the resolution of the disease in children��s mastocytosis. The finding of these mutations may have important implications for pathogenesis understanding, Anemarsaponin-BIII prognosis and therapeutics particularly regarding the potential use of c-kit inhibitors. To our knowledge, no large study has attempted to describe characteristics of adult��s mastocytosis according to the age of disease��s onset. In this report, we compared phenotype and c-kit genotype in a large prospective cohort of adults�� patients with histologically confirmed mastocytosis according to their childhood or adulthood age of onset.