Abstract： Background The FIP1L1-PDGFRα fusion gene plays an important role in the pathogenesis of chronic eosinophilic leukemia (CEL) and is a direct therapeutic target of the tyrosine kinase inhibitor imatinib mesylate.Methods In 24 hypereosinophilic syndromes (HES) patients, using reverse transcriptase-polymerase chain reaction (RT-PCR), nested PCR and sequence analysis, we investigated the frequency of FIPIL1-PDGFRα and other abnormalities of tyrosine kinase family genes like PDGFRα, PDGFRβ, C-KIT, FGFR1, ABL and FLT3 as well as gene mutation "hotspots", like MPL515 and JAK2V617F, frequently involved in myeloproliferative diseases. Fluorescence in situ hybridization was used to confirm the 4q12 deletion.Results The FIPILI-PDGFRα fusion transcript was found in 8 (33%) of 24 patients with HES, corresponding to the chromosome 4q12 deletion identified by FISH. The FIP1L1-PDGFRα-associated patients diagnosed with CEL, frequently had hepatosplenomegaly, eosinophil-related tissue damage, anemia, thrombocytopenia, myelofibrosis and a short overall survival time. Nevertheless, imatinib mesylate induced rapid and complete hematological responses in treated FIP1L1-PDGFRα cases, followed by molecular remission and reversal of myelofibrosis. FIP1L1-PDGFRα fusion could co-exist with other mutations of tyrosine kinase family genes, like FLT3 or PDGFRβ. We also demonstrated that the SNPs of PDGFRβ were associated with selective splicing of exon 19 in case 20.Conclusions Correlating the CEL genotype with phenotype, FIPIL 1-PDGFRα emerges as a relatively homogeneous clinicobiological entity that co-exists with other abnormalities of tyrosine kinase family genes. It reflects the disease progression and there is a good response to imatinib. Detection of the FIP1L1-PDGFRα fusion gene is valid for both CEL diagnosis and therapy surveillance.