OBJECTIVE In polycythemia vera (PV) there is no specific clonal marker because the molecular lesion responsible for PV is unknown. The recent demonstration that the PRV-1 gene is overexpressed in granulocytes from patients with PV provided the rationale for the current study to investigate whether PRV-1 is structurally rearranged, thus explaining its aberrant expression. MATERIALS AND METHODS Fluorescence in situ hybridization was used to determine chromosomal localization of PRV-1 and to study whether the PRV1 gene is rearranged in 26 patients with PV. RESULTS PRV-1 was localized to chromosome 19, band region q13.12-2. Structural rearrangements of PRV-1 were evaluated in bone marrow cells from 26 patients with PV: 14 with a normal karyotype and 12 with an abnormal karyotype. None of 150 metaphase cells or more than 10,000 interphase cells demonstrated PRV-1 gene deletion, amplification, or separation of the probe signal, which would indicate a PRV-1 rearrangement. CONCLUSION These findings are consistent with a lack of structural rearrangement of PRV-1 in patients with PV. Thus, overexpression of PRV-1 in granulocytes from patients with PV is related to mechanisms that do not involve structural genetic changes.