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to nINTRODUCTION Transplantation of hematopoietic stem cells into immunosup-pressed recipients has been used over the past three decades as a treatment for diseases such as leukemia (Machida et al., 1999). In animal models, hematopoietic stem/progenitor cell (HSPC) activity of a given population is assessed by analyzing multilineage engraftment of bone marrow or peripheral blood of recipient animals (Uchida et al., 1994). Recently, tumor stem cells have been shown to exist for breast cancer (Al-Hajj et al., 2003) and medulloblastoma (Singh et al., 2004). These rare cells, which are thought to form the seed from which all other tumor cells arise, have been isolated by selecting for cells (i.e., CD133+) that can be repeatedly propagated in serial transplan-tation studies (Piccirillo et al., 2006). It is evident that tumor cells The characteristic adult pigmentation pattern of the zebrafish consists of three distinct classes of pigment cells arranged in stripes (Figure 1A): black melanophores, reflective iridophores, and yellow xanthophores (Rawls et al., 2001). The nacre mutant (Figure 1B) has a complete lack of melanocytes due to a mutation in the gene encoding the mitfa gene. (Lister et al., 1999). The roy orbison (roy) zebrafish (Figure 1C) is a spontaneous mutant and has a complete lack of iridophores, uniformly pigmented eyes, sparse melanocytes, and a translucency of the skin. The gene responsible for this mutant phenotype is currently unknown. Fish that are doubly mutant for nacre and roy are shown in Figure 1D. This line, which we have named casper for its ghost like appearance, demonstrates a complete lack of all melano-cytes and iridophores in both embryogenesis and adulthood. Most strikingly, the body of the fish is almost entirely transparent.Cell Stem Cel