NTRK1 and osteosarcoma: Human monocytes/macrophages express TrkA and p75NTR and exhibit ligand−form–dependent phenotypes; flow cytometry for TrkA/TrkB/p75NTR on CD45+ subsets, coupled to ex vivo stimulation with mature NGF versus proNGF, can profile receptor competence and functional polarization states relevant to osteosarcoma lesions (62–64).