Large-scale vertebrate chemical screening made possible by zebrafish. Embryos are incubated in groups of 5-10 with approximately 2,500 different chemicals. At 36 hours post-fertilization, in situ hybridization is conducted to analyze the expression of early hematopoietic markers such as runx1 and c-myb. The embryos are then scored for a change in hematopoietic expression. We recently used this technique  to identify 82 compounds that influence hematopoietic stem cell differentiation, the most prominent of which was dimethyl prostaglandin E2 (dmPGE2). Modified with permission, from .