CD34 and leukemia: Although the above experiments with leukemia cells (JK-GFP cells) demonstrate the feasibility of human hematopoietic cells engraftment, proliferation, and colonization of hematopoietic organs in WT zebrafish embryos for up to 7 d.p.T., the success rate of the process was very low (2/92) and it was never observed with primary human CD34-GFP cells (n = 124) that survive only 2 days after injection within the zebrafish embryo (Fig. S2M, arrow and Video 3), as previous described [19, 27].