Schematic representation of the JAK2 gene with the wild-type sequence of amino acids encoded by exon 12, and the exon 12 mutations detected so far. Regions of rearrangement are highlighted in yellow, while mutations are shown in red; the red rectangles indicate the duplicated sequences. All mutations so far identified in patients with PV or idiopathic erythrocytosis are illustrated: those previously reported^12-17  are marked with an asterisk. Blue rectangles indicate the 3 types of mutations: (i) those involving a K539L substitution; (ii) those involving a deletion of glutamic acid 543 (E543del); (iii) and duplications involving a substitution of phenylalanine 547. On the right, for each mutation the number of cases reported so far is indicated. Information on JAK2 domains is from URL: http://smart.embl-heidelberg.de.