Severe congenital neutropenia (SCN) is an inherited disorder of myelopoiesis, characterized by a maturation arrest at the stage of promyelocytes and myelocytes in bone marrow, and absence or low levels of mature neutrophil granulocytes in peripheral blood. Recently, studies of patients with SCN who subsequently developed acute myeloid leukemia (AML) revealed nonsense mutations in the cytoplasmic domain of the granulocyte colony-stimulating factor (G-CSF) receptor messenger RNA. We focused our interest on the G-CSF-mediated signaling cascade to examine the consequences of the observed point mutations for the nuclear translocation of the transcription factors Stat1 and Stat3. Expression vectors encoding for truncated G-CSF receptors were transfected in the murine fibroblast cell line C243 expressing a fusion protein consisting of the transcription factor Stat1 and Stat3, respectively, and the green fluorescent protein (GFP). Nuclear translocation of the GFP fusion proteins was examined after G-CSF stimulation of the transfected cells.