No specific diagnostic markers have been described in essential thrombocythemia (ET). PRV-1 (polycythemia rubra vera-1), TPO (thrombopoietin), and c-MPL (myeloproliferative leukemia virus oncogene) genes are candidate ET molecular markers because of their implication in the pathogenesis of ET. We have studied the status of PRV-1, TPO, and c-MPL genes in 30 ET patients by a fluorescence in situ hybridization (FISH) technique using three noncommercial, locus-specific probes for PRV-1 (BAC RP11-160A19, located at 19q13.2), TPO (BAC RP11-45NP16, located at 3q27), and c-MPL (BAC RP11-297L5, located at 1p34). FISH study showed no PRV-1, TPO, and c-MPL cytogenetic abnormalities in any of the analyzed cases. Our results suggest a lack of structural and numerical rearrangements (deletions, translocations, or amplifications) of PRV-1, TPO, and c-MPL genes in ET patients.