A recently described atypical myeloproliferative disorder is invariably associated with reciprocal translocations involving 8p11-12. The most common rearrangement is a t(8;13)(p11;q11-12). Here we determine that this translocation results in the fusion of the fibroblast growth factor receptor 1 gene (FGFR1), a member of the receptor tyrosine kinase family at 8p11, to a novel gene at 13q11-12 designated RAMP . The predicted RAMP protein exhibits strong homology to the product of a recently cloned candidate gene for X-linked mental retardation, DXS6673E . We also provide the first report of a novel, putative metal-binding motif, present as five tandem repeats in both RAMP and DXS6673E. RT-PCR detected only one of the two possible fusion transcripts, encoding a product in which the N-terminal 641 amino acids of RAMP become joined to the tyrosine kinase domain of FGFR1. Receptor tyrosine kinases are not commonly involved in the formation of tumour-specific fusion proteins. However, the previous reports of involvement of receptor tyrosine kinases in fusion proteins in non-Hodgkin's lymphoma, chronic myelomonocytic leukaemia and papillary thyroid carcinoma described similar rearrangements. By analogy with these, we propose that the RAMP-FGFR1 fusion product will contribute to progression of this myeloproliferative disorder by constitutive activation of tyrosine kinase function.