Abstract: We report a new case of blastic plasmacytoid dendritic cell neoplasm (BPDCN) with extensive immunophenoptyping, genotyping (karyotype, array-comparative genomic hybridization, and fluorescent in situ hybridization), and long-term tumor cells culture. BPDCN is a very rare and aggressive disease clinically characterized by a skin revealing localization more or less rapidly disseminating to the bone marrow and other organs with or without and leukemia. The disease was initially phenotypically characterized by the expression of both CD4 and CD56 antigens, whereas lymphoid and myeloid lineage antigens were negative. A phenotypic link with alpha-interferon (IFN-I)-producing plasmacytoid dendritic cells was demonstrated. The data collected in this case report provide additional biological and genotypical data on tumor cells of BPDCN. This study confirms the capability of tumor cells to secrete IFN-I, demonstrated by biological IFN-I activity of cultured cells and immunohistochemical expression of Mx-1 protein. Although a common genetic profile involving chromosomes 5, 6, 9, 12, 13, and 15 has been identified, no specific genetic marker has been demonstrated that is specific to BPDCN. The demonstration of ETV6 gene deletion in this case deserves further investigations as a putative BPDCN marker.