PU.1 is a member of the Ets family of transcription factors and is required for the development of multiple hematopoietic lineages. It plays a pivotal role in normal myeloid differentiation, and regulates the expression of immunoglobulin and other genes that are important for B-cell development. Antibody against PU.1 stains B lymphocyte in germinal center and mantle B cell, but not plasma cell. Anti-PU.1 labels many types of B cell lymphoma including mantle cell lymphoma, but PU.1 is not expressed in classical Hodgkin lymphoma (cHL). The lack of transcription factor PU.1 protein expression in cHL, a lymphoproliferative disease of predominantly B-cell origin, likely contributes to the lack of immunoglobulin expression and incomplete B-cell phenotype characteristic of the Reed-Sternberg cells in cHL.