Mast cell disorders constitute a heterogeneous group of diseases, including : * mastocytosis, i.e. cutaneous, indolent and severe forms of the disease, such as aggressive mastocytosis and mast cell leukemia) ; * mast cell-associated diseases such as mast cell activation syndrome (idiopathic, secondary or clonal), affecting both children and adults. No epidemiological data are currently available in France. In France, medical care of mast cell disorders is mainly provided by a rare disease network (CEREMAST), whose CRMR is located at the Necker Enfants Malades hospital in Paris. A total of 20 centers are located throughout France. Our aim is to use this network to study patients suffering from these diseases. The overall aim of the study is to improve the understanding, diagnosis, prognosis, recognition and management of patients with mastocytosis.