Myeloperoxidase (MPO) is present in azurophilic granules which appear in the promyelocyte stage of differentiation, and is the most common functional protein of myeloid cells. With progress in molecular biology, the expression and regulation of MPO have been clarified in normal myeloid and leukemic cells, not only by enzymatical activity but at the gene level MPO expression is affected by the differentiation of myeloid cells, and has been suggested to be regulated by myeloid cell growth factors, such as granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor and interleukin-3. In the past decade the signal transduction from their receptors has been clarified. This review describes the expression and regulation of the MPO gene in myeloid cells including myeloid disorders, such as myeloid leukemia or myelodysplastic syndromes, The effects on MPO by myeloid growth factors and signal transduction from their receptors are also presented.