Box 1. Hematopoietic development in zebrafish As in other vertebrates, zebrafish hematopoiesis develops in sequential waves (Davidson and Zon, 2004). Primitive hematopoiesis starts at two anatomically separate mesodermal sites in the embryo: the intermediate cell mass, which contributes to the first circulating erythrocytes, and the rostral blood island, which gives rise to primitive macrophages and neutrophils (Detrich et al., 1995; Palis and Yoder, 2001). A second transient hematopoietic wave occurs from the posterior blood islands, where multipotent erythromyeloid progenitors are generated between 24 and 30 hpf (Bertrand et al., 2007, 2010b). Between 28 and 32 hpf, definitive hematopoietic stem/progenitor cells (HSPCs) start emerging from the ventral dorsal aorta – the equivalent of the mammalian aorta-gonad-mesonephros region (Bertrand et al., 2010a, 2010b; Kissa and Herbomel, 2010). These definitive HSPCs then migrate to and amplify in the caudal hematopoietic tissue (Bertrand et al., 2010a; Boisset et al., 2010) – a site equivalent to the fetal liver in mammals – before they subsequently colonize the thymus and the kidney marrow. The latter is the adult hematopoietic organ and sustains hematopoiesis throughout the zebrafish life span (Chen and Zon, 2009; Jin et al., 2007), and the thymus enables T-cell maturation. Box 2. Methods, advantages and disadvantages for modeling hematological disorders in zebrafish Methods Transient strategies: mRNA or cDNA injections for overexpression of target genes, morpholino oligonucleotide (MO) injection for downregulation. MOs are nonionic DNA analogs in which the ribose moiety has been substituted with an MO ring. They are generally designed to be complementary to the translational start site or a specific splice site in the pre-mRNA of the target gene, preventing translation or splicing of the pre-mRNA by a steric blocking mechanism. The technique is based on injecting these modified oligonucleotides, which then prevent expression of the targeted gene (see also https://www.gene-tools.com). Recently, serious concerns have been raised as to the specificity of MO effects (Kok et al., 2015). However, adequately controlled MOs used according to specific guidelines should still be accepted as a generic loss-of-function approach in the absence of genetic evidence (Blum et al., 2015; Stainier et al., 2017). Permanent strategies: transgene expression, which allows expression of human sequences or fusion reporters, and genome editing tools. Zinc-finger nucleases (ZFNs) can be used for targeting a unique genomic locus. Transcription activator-like effector nucleases (TALENs) are suitable for knock-in strategies or for removing large spans of DNA to cause genomic deletions. TILLING (targeted induced local lesions in genomes) allows directed introduction of point mutations in a specific gene. The CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 system can cut at a specific location but also allows knock-ins or removal of existing genes/sequences (Phillips and Westerfield, 2014). Xenotransplantation of human cancer cells (Konantz et al., 2012; Parada-Kusz et al., 2018; Veinotte et al., 2014) to generate patient-derived xenograft models, which may allow targeted therapy development and disease outcome prediction (Bentley et al., 2015; Gacha-Garay et al., 2019). Advantages High fecundity, small size and fast embryonic development, which make the zebrafish amenable for large-scale screens. Ex utero fertilization and development, which allows (genetic) manipulation at all developmental stages and analyses of phenotypes that would die in utero in mice. Transparency during development and in adult casper (White et al., 2008) or tra/nac (Krauss et al., 2013; Lister et al., 1999) fish, which allows live imaging of hematopoietic cells. Conserved regulatory pathways, especially in hematopoiesis. Disadvantages Duplicated genome with many single-nucleotide polymorphisms and insertion/deletion variations. Cold-blooded animal – evolutionarily far away from humans. Lack of certain organs (e.g. lung, breast). Lack of specific antibodies for experimental work. Fish are greatly influenced by their environment (temperature, density etc.). Different morphology of certain blood cells. Lack of a fully functioning adaptive immune system.