Sema4D, the ligand for Plexin-B1, is expressed by cells in the prostate stroma and tumor-associated macrophages (46) and Sema4D secreted from the tumor microenvironment may act as a repellent cue to inhibit migration and invasion of tumor cells expressing WT Plexin-B1, confining the tumor cells to the primary tumor mass. The gene discussed is SEMA4D; the disease is neoplasm.