The present invention relates to plexin D1 for use as a targetable protein in the treatment or diagnosis of disorders that involve expression of plexin D1. Diagnosis is suitably effected by detecting the presence of plexin D1 in the body or a bodily tissue or fluid, whereas treatment is effected by targeting plexin D1 for delivery of therapeutics to the site where treatment is needed. The invention further relates to the use of molecules that bind plexin D1, a nucleic acid encoding plexin D1 or a ligand of plexin D1 for the preparation of a therapeutical composition for the treatment or diagnosis of disorders that involve expression of plexin D1. The disorders comprise disorders in which plexin D1 is expressed on tumor cells, tumor blood vessels or activated macrophages.