CD4 and acute lymphoblastic leukemia: Radiolabeling yielded high radiochemical purity (≥95%) and specific binding of 64Cu-hCD4-Nb1 to CD4-expressing HBP-ALL cells (46.5% ± 5.6%) in vitro that was ~30 times higher than the non-specific binding to CD4-negative DHL cells or of the radiolabeled 64Cu-GFP-Nb control (Supplementary Figure 12A).