We report the fusion of the Huntingtin interactin protein 1(HIP1) gene to the platelet-derived growth factor βreceptor (PDGFβR) gene in a patient with chronic myelomonocytic leukemia (CMML) with a t(5;7)(q33;q11.2) translocation. Southern blot analysis of patient bone marrow cells with a PDGFβR gene probe demonstrated rearrangement of the PDGFβR gene. Anchored polymerase chain reaction using PDGFβRprimers identified a chimeric transcript containing the HIP1gene located at 7q11.2 fused to the PDGFβR gene on 5q33. HIP1 is a 116-kD protein recently cloned by yeast two-hybrid screening for proteins that interact with Huntingtin, the mutated protein in Huntington's disease. The consequence of t(5;7)(q33;q11.2) is an HIP1/PDGFβR fusion gene that encodes amino acids 1 to 950 of HIP1 joined in-frame to the transmembrane and tyrosine kinase domains of the PDGFβR. The reciprocalPDGFβR/HIP1 transcript is not expressed. HIP1/PDGFβR is a 180-kD protein when expressed in the murine hematopoietic cell line, Ba/F3, and is constitutively tyrosine phosphorylated. Furthermore, HIP1/PDGFβR transforms the Ba/F3 cells to interleukin-3–independent growth. These data are consistent with an alternative mechanism for activation of PDGFβR tyrosine kinase activity by fusion with HIP1, leading to transformation of hematopoietic cells, and may implicate Huntingtin or HIP1 in the pathogenesis of hematopoietic malignancies.