Translocations affecting FGFR1 are hallmarks of the 8p11 myeloproliferative syndrome (EMS), also known as stem cell leukemia/lymphoma syndrome, an aggressive stem cell myeloproliferative neoplasm that is associated with eosinophilia, poor prognosis, T-cell lymphoma, and frequent progression to acute myeloid leukemia. The most common translocation detected in EMS is t(8;13)(p11.2;q12.1) fusing FGFR1 to ZMYM2 (a.k.a. ZNF198). Several other rearrangements affecting the FGFR1 locus are also common in EMS, all of which result in fusion proteins comprising the tyrosine kinase domain of FGFR1 and a dimerization domain of a partner protein. Due to dimerization these fusion proteins show constitutive kinase activity. Currently, bone marrow or stem cell transplantation is the only curative treatment for patients with EMS. In vitro studies suggest that certain receptor tyrosine kinase inhibitors may provide a new therapeutic option. Detection of FGFR1 rearrangements using FISH may assist in the diagnosis of patients with this aggressive stem cell disorder.