The FOXP1 protein belongs to a functionally diverse family of winged-helix or forkhead transcription factors that have diverse roles in cellular proliferation, differentiation, and neoplastic transformation. The FOXP1 gene has been mapped to chromosome 3p14.1, a region that commonly shows loss of heterozygosity in a wide range of tumors and is reported to contain a tumor suppressor gene(s). The FOXP1 protein is widely expressed in normal human tissues. It labels activated B cells in the mantle zone and germinal center of lymphoid tissues. In lymphoid malignancies, FOXP1 protein expression may be found in diffuse large B-cell lymphomas and extranodal marginal zone B-cell lymphomas of mucosa-associated lymphoid tissue (MALT). Strong expression of FOXP1 is associated with poor disease-free survival and transformation to diffuse large B- cell lymphomas. Recently, studies suggested a role of FOXP1 in the regulation of ER expression. FOXP1 expression is correlated with ER expression and improved survival in breast cancer patients. Nuclear expression of FOXP1 is associated with ER expression, while cytoplasmic expression of FOXP1 is linked to myometrial invasion in endometrial cancer.