SEMA4D/CD100 was found to be expressed in B-cell chronic lymphocytic leukemia (CLL) cells and in normal CD5+ B lymphocytes, and its high-affinity receptor Plexin-B1 was found in BM stromal cells, follicular dendritic cells, and activated T lymphocytes. The gene discussed is PLXNB1; the disease is B-cell chronic lymphocytic leukemia.