FLRT2 and melanoma: Because Flrt2 acts primarily as a repulsive ligand for the Unc5b receptor (14, 16), we injected endothelial cell–specific Unc5b-knockout mice (Cdh5-BAC-CreERT2Unc5bfl/fl; referred to hereafter as Unc5biΔEC mice) with B16 melanoma cells.