Lifelong hematopoiesis involves the proliferation and differentiation of successive clones into mature blood lineages. Mono- or oligoclonal hematopoiesis can occur in elderly patients who have somatic TET2 or DNMT3A mutations. The production of blood by one or a few clones with somatic mutations renders patients susceptible to blood disorders such as refractory cytopenias or myelodysplasia. It is unclear which combinations of these mutations are sufficient to promote clonal dominance or maintain malignancy. Here, we have developed a system in the zebrafish to study the genetics of clonality. A Zebrabow fish allows for clonal labeling of stem cells and their progeny with random combinations of three fluorescent proteins, providing enough diversity to produce up to 30 detectable colors. Using a tissue-specific inducible CreERT2 system, we labeled stem cells at 36 hpf of development and then grew the animal to adulthood. This labeled roughly 20 clones of cells, establishing that there are 20 cells of the developing aorta that contributed to adult hematopoiesis. We have also developed tissue-specific CRISPR technology to target multiple genes simultaneously. Using this approach, we will look for gene combinations that are required for clonal hematopoiesis. We then will undertake a chemical screen to find pathways that suppress clonal dominance. Single cell analysis with the Tenen and Orkin labs will be used to examine specific gene expression in particular dominant clones after FACS isolation. We plan to evaluate the activity of the genes and small molecules discovered using the zebrafish in the mouse Hue system (Scadden) and the transposon barcoding strategy (Camargo). Our collaboration with Dr. Orkin will identify chromatin factors required for the establishment of normal clonal dynamics. Our studies will have an impact on understanding early clonal events that regulate normal and malignant hematopoiesis, and could develop novel therapeutics to suppress clonal hematopoiesis.