Summary. Translocations affecting the chromosomal locus 8p12 are hallmarks of an atypical stem cell myeloproliferative disorder. These events disrupt the fibroblast growth factor receptor 1 (FGFR1) gene and fuse the FGFR1 C-terminus catalytic domain with unrelated proteins. Here, we report on the characterization of the 19q13.3 locus as the fifth FGFR1 chromosomal partner.Keywords: myeloproliferative disorder, translocation, FGFR1 gene, chromosome 8, chromosome 19.The FGFR1 gene encodes one of the four tyrosine kinase receptors for members of the fibroblast growth factor family and its disruption is the hallmark of a stem cell myeloproliferative disorder (MPD) linked to the 8p12 chromosomal region . This MPD has been already associated with four reciprocal translocations t(6;8)(q27;p12), t(8;9)(p12;q33), t(8;13)(p12;q12) and t(8;17)(p12;q25). Three FGFR1 partner genes have been cloned, FOP at 6q27 (Popovici et al, 1999), CEP110 at 9q33 (Guasch et al, 2000) and FIM/ZNF198 at 13q12 Xiao et al, 1998). The 17q25 gene has not been identified to date (Sohal et al, 1999). In all cases analysed to date, the FGFR1 breakpoints are tightly clustered in the 1´2 Kb intron 8 or near the exon 8/intron 8 junction. The chimaeric proteins with putative oncogenic properties contain potential oligomerization domains encoded by the 6q27 (FOP), 9q33 (CEP110) and 13q12 (FIM) genes fused to the tyrosine kinase domain of FGFR1. We have recently shown that the three fusion proteins are delocalized from their normal counterparts, display constitutive kinase activity triggered by dimerization mediated by the protein± protein interaction motifs of the FGFR1 protein partner and promote survival of pro-B Ba/F3 cells after IL-3 withdrawal (Ollendorff et al, 1999;Guasch et al, 2000). These accumulating data suggest that a common mechanism may sustain the oncogenic activity of the rearranged kinase.A 70-year-old man presented to the Dijon Hospital with a dermatological and acute renal disease paraneoplastic syndrome. Haemoglobin was 17 g/dl and the leucocyte count was 3´9 Â 10 9 /l, with 5% myelocytes and 7% blast cells. Platelet count was 140 Â 10 9 /l. The bone marrow aspirate showed 37% of peroxidase-negative blast cells, 25% of erythroblasts with dyserythropoiesis, abnormal megakaryocytes and 5% of eosinophiles (Fig 1A and B respectively). Immunophenotype was studied on fresh bone marrow mononuclear cells by flow cytometry. Blast cells were CD45 low and displayed cytoplasmic CD13 and myeloperoxidase. All lymphoid and other myeloid markers were negative. A diagnosis of acute myeloid leukaemia (AML) M0 probably secondary to a myeloproliferative disorder was made. The patient was treated with rubidazone/aracytine for 6 months and died 15 months later from increased renal disease. Cytogenetic study of the bone marrow revealed the karyotype abnormality t(8;19)(p12;q13.3) associated with loss of the Y chromosome in 14 out of 15 metaphases.The 8p12 MPD is characterized by common clinicopathological features: B-or T-cell lymphoblastic leukaemia/ lym...