After 2–3 weeks, mice bearing HPB-ALL xenografts were intravenously (i.v.)injected with 5 μg of CD4-Nb1-Cy5.5, CD4-Nb4-Cy5.5, or a control Nb (GFP-Nb-Cy5.5) and non-invasively in vivo investigated by optical imaging (OI) in intervals over the course of 24 h (Figure 5A, Supplementary Figure 10A). Here, CD4 is linked to acute lymphoblastic leukemia.