Dear Editor, A 66-year-old female was diagnosed in 2000 with hairy cell leukemia (HCL). Flow cytometry from her bone marrow biopsy (BMB) demonstrated a monotypic B cell population expressing CD11c, CD22, CD25, and CD103. She was successfully treated with cladribine [1]; however, in 2003, she relapsed, and BMB showed a monotypic B cell population. Cytogenetic analysis revealed an abnormal hyperdiploid karyotype in 2 of 20 metaphase cells: 48,XX,+4,del(6)(q23), del(8)(p21)x2,+12,del(14)(q24), add(17)(p13) (Fig. 1). She was again treated with cladribine and rituximab [2]. In 2008, imaging revealed splenomegaly. Splenectomywas performed, and pathology showed HCL. BMB revealed extensive involvement by a monotypic B cell population negative for CD5, CD10, CD103, and CD25. This phenotype was the same as reported on previous BMB, but differed in that it was CD25 and CD103 negative. Five months later, the patient presented with a left pyriform sinus mass (SUV of 32.8 on PET scan) which was biopsied (Fig. 2). BMB revealed involvement by the previously diagnosed HCL with a monoclonal B cell population demonstrating a phenotype similar to that observed in previous studies. Based on histologic and immunophenotypic findings, this tumor was classified as Langerhans/dendritic cell sarcoma (L/DCS) (expression S100, CD1a; negative for B and T cell markers and CD30). Cytogenetic evaluation identified the hyperdiploid complex karyotype: 47,XX,+4,del(6)(q23),del(8)(p21)x2,+12, -13,del(14)(q24),add(17)(p13). This karyotype and the one obtained in 2003 BMB positive for HCL were essentially identical, although L/DCS presented with monosomy 13. In addition, clonal immunoglobulin gene rearrangements identified by polymerase chain reaction from both specimens were also identical. Combined, cytogenetic, and immunoglobulin gene rearrangement studies suggest that both the HCL and L/DCS developed from the same precursor cells. The patient was treated with three cycles of gemcitabine and docetaxel [3]. However, due to significant progression, the patient opted for hospice. Clinical data have shown that two hematopoietic populations in the same patient may share identical genetic changes or abnormalities, raising the possibility that tumors expressing the phenotype of one hematopoietic lineage might “transdifferentiate” into a genetically similar but phenotypically distinct tumor of a different lineage [4]. Histiocytic/dendritic cell sarcomas (H/DCS) arising from follicular lymphoma (FL) have been reported earlier (eight patients) [5]. In six of these patients, both lesions possessed the same genetic aberration, namely the FL-associated IGH/BCL2 gene rearrangement associated with the t(14;18)(q32;q21). The remaining two demonstrated identical BCL2/JH or clonal IGH gene rearrangements or both. Based on these observations, the authors suggested a clonal relationship between FL and H/DCS. Feldman et al. also demonstrated a common clonal origin in a patient with acute lymphoblastic leukemia and subsequent histiocytic sarcoma, based on identical IGH and TCRγ gene rearrangements in both lesions [6]. Zhang et al. described a Electronic supplementary material The online version of this article (doi:10.1007/s00277-011-1399-5) contains supplementary material, which is available to authorized users. A. Muslimani :M. M. Chisti (*) : I. Boxwala : I. Jaiyesimi Department of Hematology and Oncology, Oakland University William Beaumont School of Medicine, William Beaumont Hospital, 3577 W 13 Mile Rd, Suite 202, Royal Oak, MI 48073, USA e-mail: mohsinchisti@yahoo.com