HuXBR1-402 targets proliferating leukemia cells in vivo. Briefly, 5 × 10⁶ hROR1⁺ TLC1 murine CLL splenocytes were engrafted via tail vein injection into 8-week-old C57BL/6 mice. After establishment of leukemia (peripheral leukemia proportion is >5% of total white blood cells), mice were treated 3 times a week for 1 week with 1 mg/kg huXBR1-402-PNU or relevant controls. Three days after the last treatment, mice were pulsed with EdU (50 µg/kg) and killed 24 hours later to obtain the following data: numbers of EdU⁺ CD19⁺/CD5⁺ leukemia B cells (A) from splenocytes, obtained with flow cytometry (n = 4 mice per group); spleen weight (grams) (B); and fold change (C) in the numbers of peripheral blood leukemia cells. Mice were bled 4 days before euthanasia, and blood was again collected at time of euthanasia. Fold change in leukemia numbers are calculated by (peripheral leukemia cell count at death)/(peripheral leukemia cell count 4 days before). Panels B and C used n = 5 mice per group. Statistical significance for multiple comparisons was analyzed via one-way analysis of variance with Tukey’s post hoc test. ns, not significant; Tras-G5-PNU, trastuzumab-G5-PNU. Error bars indicate ± SE.