We searched for JAK2 exon 12 mutations in patients with JAK2 (V617F)-negative myeloproliferative disorders. Seventeen patients with polycythemia vera (PV), including 15 sporadic cases and 2 familial cases, carried deletions or duplications of exon 12 in circulating granulocytes but not in T lymphocytes. Two of the 8 mutations detected were novel, and the most frequent ones were N542-E543del and E543-D544del. Most patients with PV carrying an exon 12 mutation had isolated erythrocytosis at clinical onset, unlike patients with JAK2 (V617F)-positive PV, most of whom had also elevations in white blood cell and/or platelet counts. Both patients with familial PV carrying an exon 12 mutation had an affected sibling with JAK2 (V617F)-positive PV. Thus, several somatic mutations of JAK2 exon 12 can be found in a myeloproliferative disorder that is mainly characterized by erythrocytosis. Moreover, a genetic predisposition to acquisition of different JAK2 mutations is inherited in families with myeloproliferative disorders.