The higher susceptibility for thrombocytosis during the neonatal period may result from various physiological phenomena: a high Tpo gene expression in the bone marrow during the ontogeny of medullary haematopoiesis [13], higher circulating Tpo concentrations in fetuses and neonates than in children and adults [14], and an increased sensitivity of megakaryocytic progenitor cells to Tpo. This evidence concerns the gene TPO and Thrombocytosis.