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Much of our understanding of the molecular anomalies involved in the process of oncogenesis has resulted from research into malignant hematologic diseases, facilitated by the accessibility of hematopoietic cells. For example, in lymphoid tumors, rearrangement of the genomic DNA can lead to the juxtaposition of proto-oncogenes and the highly active sequences regulating synthesis of immunoglobulins or T-cell receptors. The subsequent malignancy results from an uncontrolled overexpression of a normal protein. This type of "quantitative" anomaly occurs in follicular lymphomas where B-cells overexpress the normal BCL2 protein which inhibits apoptosis, contributing to immortalization of the B done. The same type of rearrangement process can approach gene fragments which fusion and lead to production of a highly oncogenic chimerical or truncated abnormal protein. Such "qualitative" anomalies occur in myeloid hemopathies. Both types of anomalies involve genes controlling the cell cycle, cell differentiation or cell death (apoptosis), in particular transcription factors (for example, E2A, RARA, MYC) and molecules involved in signal transduction (for example RAS, ABL, LCK). A molecular anomaly can be detected in approximately 30% of all cases of acute leukemia and in up to 75% of the non-Hodgkin lymphomas. Analysis of the junction fragments of the different heavy chains of the immunoglobulins produced in these cases provides a specific marker for detecting the B or T-cell clone in digestive or skin biopsies. For example, detection of a BCR-ABL transcript in a patient with primary thrombocythemia or an atypical myeloproliferative syndrome can be diagnostic and detection of the donal immunoglobulin or T-cell receptor rearrangement can confirm the malignant nature of the lymphoid proliferation. Molecular markers also have prognostic value allowing patient stratification and more adapted therapy. Molecular anomalies detected in malignant hematologic diseases are thus examples of nearly perfect "tumor-specific" markers.

Richter's syndrome is a large-cell lymphoma occurring in patients with chronic lymphoid leukaemia (CLL). It is rarely limited to cutaneous locations. We report the case of a 61-year-old patient with CLL who developed multiple skin lesions due to immunoblastic lymphoma.Six years after onset of CLL controlled by chloraminophen, a tumoural syndrome developed and was treated by chemotherapy. A papulonecrotic cutaneous eruption was then observed on the face, the presternal area and then on the fingers. Skin biopsy revealed large-cell CD20+ CD30+ immunoblastic lymphoma. Polymerase chain reaction on the heavy chain immunoglobulins showed a band with the same size in the skin biopsy and in the circulating cells. No transformation was found in lymph node and bone marrow biopsies. The lesions disappeared after 4 treatments with VP16, cysplatin and methylprednisolone. CLL was again stabilized after the first chemotherapy sessions.Richter's syndrome is rarely discovered due to skin lesions. There have only been 4 similar cases reported in the literature. The clinical presentation of the lesions is insufficient to distinguish them from manifestations of CLL. Histological examination of the biopsy is required to make the diagnosis of high grade lymphoma. Prognosis is usually poor after discover of this type of lymphoma. This is the first observation of Richter's syndrome revealed by a skin lesion in which polymerase chain reaction suggested that the skin lymphoma and the CLL cells came from the same B clone.