FGFR1 translocations are hallmarks of 8p11 myeloproliferative syndrome (EMS), also known as stem cell leukemia/lymphoma syndrome, an aggressive stem cell myeloproliferative neoplasm associated with eosinophilia, T-cell lymphoma, and frequent progression to acute myeloid leukemia. T(8;13)(p11.2;q12.1) is the most common rearrangement in EMS, resulting in fusion of FGFR1 to ZMYM2 (a.k.a. ZNF198). Other FGFR1 rearrangements are also prevalent in EMS, and all generate fusion proteins composed of the tyrosine kinase domain of FGFR1 and a dimerization domain of a partner protein, leading to constitutively activated 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.