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Bifurcation is an essential feature of biology. It underlies differentiation as one cell, through a process of mitosis accompanied by altered gene expression, forms two distinct cell lineages. The hematopoietic system is a dramatic example of this phenomenon. A single lymphohematopoietic stem cell, can over the course of several bifurcations, differentiate and then mature into at least 11 unique functional cells. In some cases, these cells can mature further into different phenotypes influenced by the environment in which they reside. Consider, for example, the monocytes, Kupffer cells, osteoclasts, microglia, and alveolar macrophages.

One of the critical points of hematopoietic bifurcation is the differentiation of the lymphohematopoietic stem cell into the common myeloid and common lymphoid progenitor. It is at this point that differentiation into these distinct lineages separates hematology into two specialized areas of research and clinical practice: the myeloid and lymphoid neoplasms. Unlike most of the maturing myeloid cells, the lymphoid cells do not lose their mitotic capability. This requirement for continued replication and repair of DNA, along with the rearrangements required of immunoglobulin and T-cell receptor genes during maturation, provides the risk of neoplastic gene mutations; these requirements result in a panoply of lymphocytic neoplasms, grossly divided into B-lymphocyte, T-lymphocyte, and natural killer cell tumors. The complexity of this array is extensive, with over 70 specific lymphocytic tumors in the 2016 World Health Organization classification of lymphocytic malignancies.

The lymphoid neoplasms are the subject of this text. Neoplasms originating in the lymphoid progenitor cell hierarchy constitute the lymphomas and lymphocytic leukemias. These tumors afflicted over 105,000 Americans and resulted in over 23,000 deaths in 2017. Their effects worldwide are dramatically larger. It is these compelling numbers that prompted the editors to prepare a "breakaway" text on the malignant lymphocytic neoplasms, based on the chapters that discussed these diseases in the ninth edition of Williams Hematology. Approximately 3 years have passed since those chapters were written. The editors asked the authors of these 21 chapters to revise and update them in the light of three recent developments: an expanded classification of the lymphocytic neoplasms by the World Health Organization, advances in the understanding of biology and genetics of these tumors, and advances in therapeutic approaches to the lymphomas and lymphocytic leukemias. The authors have graciously and expeditiously done so. With their help and expertise, we can now provide a timely text that covers the lymphomas and lymphocytic leukemias.

It is hoped the reader, from the accessibility of these new versions of the chapters, will derive benefit in their research, clinical practice, and learning.

Marshall A. Lichtman
Oliver W. Press
Jonathan P. Leonard

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