Lymphoid Enhancing Factor 1 (LEF1) functions as a transcription factor within the TCF/LEF family. It acts as a regulator in Wnt signaling pathways. LEF1 plays a vital role in the development of lymphocytes and is typically found in T and pro-B cells, while it is absent in mature B cells. A strong presence of LEF1 in the nucleus has been noted in most cases of chronic lymphocytic leukemia/small lymphocytic lymphoma, and it is not found in other types of small B cell lymphomas. The use of anti-LEF1 could assist in distinguishing chronic lymphocytic leukemia/small lymphocytic lymphoma from other forms of small B cell lymphomas.