Sema4D binding to plexin-B1 also stimulates the guanine nucleotide exchange factor (GEF) activities of PDZ-RhoGEF and leukemia-associated RhoGEF bound to the plexin-B1 C terminal PDZ-binding motif, which facilitates conversion of RhoA from the GDP-bound form to the GTP-bound from (10). Here, PLXNB1 is linked to leukemia.