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Abbreviations: DN: double negative GTP: guanasine triphosphate MW: molecular weight P: postnatal day pAb: polyclonal antibody TEC: thymic epithelial cells TH: tyrosine hydroxylase Full correspondence: Dr. Anna Mondino, Lymphocyte Activation Unit, Division of Immunology, Transplantation and Infectious Diseases, San Raffaele Scientiﬁc Institute. Via Olgettina, 58, 20132 Milano, Italy Fax: 139-02-2643-4844 e-mail: anna.mondino@hsr.it Additional correspondence: Ivan de Curtis, Cell Adhesion Unit, Division of Neuroscience, San Raffaele Scientiﬁc Institute. Via Olgettina, 58, 20132 Milano, Italy e-mail: decurtis.ivan@hsr.it Fax: 139-02-2643-4813 Received: 3/8/2010 Revised: 7/2/2011 Accepted: 23/2/2011 Accepted article online: 7/3/2011 & 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu