During childhood, patients with RASopathies (Noonan syndrome and related diseases) can harbor various hematological anomalies ranging from isolated monocytosis, myelemia, thrombocytopenia or splenomegaly to myeloproliferative disorders. These anomalies may spontaneously disappear or persist, sometimes leading to juvenile myelomonocytic leukemia. Guidelines for initial screening and subsequent hematological follow-up have recently been published in France: peripheral blood analysis should be performed in all newly diagnosed patients and followed by biannual peripheral blood analysis in infants until the age of 2 years. In order to describe the characteristics of these abnormalities in terms of their incidence, age of occurrence, evolution and relation to genotype, we are conducting a longitudinal prospective study whose aim is to analyze peripheral blood cell counts and smears at diagnosis and one year later. In patients \<3 years of age recruited at certain centers, biobanking of mononuclear cells will be performed. These data could yield a new insight into hematological anomalies in patients with RASopathies and thereby help physicians to determine the appropriate rhythm for hematological follow-up according to genotype.