and inhibition via the release of various hematopoietic growth factors (eg, CSFs). Colony-stimulating factors are so called because they induce hematopoietic prog-enitor cells to form colonies in in vitro culture assays. These proteins were discovered in the 1960s in studies of cloned hematopoietic cells. There are several general rules that apply to the function of these growth factors. Various stimuli such as stress, infection, hemorrhage, and bone marrow depletion can lead to increases in growth factors with subsequent feedback to the stem cell.66,67 See Table 4 for an overview of the function of growth factors.