CD44 and neoplasm: In this work, for proof-of-concept of the functionality of NGFR-based spacers, we chose as target antigen the CD44 isoform variant 6 (CD44v6), which is knowingly over-expressed in AML (44) and MM (45), as well as on a variety of epithelial tumors (46), and has been implicated in tumor progression and resistance to radio-chemotherapy (47).