Idiopathic hypereosinophilic syndrome (HES) and chronic eosinophilic leukemia (CEL) are related hematological malignancies characterized by sustained, unexplained hypereosinophilia (>1,500 eosinophils/μL) (1–4). The termCELis used when there is evidence that the disease is of clonal origin. We recently identified theFIP1L1-PDGFRAfusion gene in approx 50% of HES/CEL cases (5). Fusion ofFIP1L1toPDGFRAis the consequence of a deletion on chromosome 4, del(4)(q12q12), with the centromeric breakpoint inFIP1L1and the telomeric breakpoint inPDGFRA. The breakpoints inFIP1L1are diverse (introns 7 to 10), but the breakpoints inPDGFRAare always in exon 12 (encoding the juxtamembrane region). because the chromosomal deletion is only 800 kb in size, it remains undetected with standard cytogenetics. In agreement with this, most patients with HES/CEL present with a normal karyotype. Here we describe three different techniques to detect the presence of theFIP1L1-PDGFRAfusion gene in peripheral blood or bone marrow cells.
