Homeobox genes encode transcription factors containing a common DNA-binding motif found in virtually all animal species. Different homeobox gene families have evolved which encode homeodomains of different types or classes and thus far approximately 170 homeobox genes have been cloned. Homeoproteins are involved in the control of animal development and several lines of evidence strongly suggest that they may contribute to the regulation of hematopoiesis. Many members of this large family are expressed in blood cells. Moreover, homeobox containing genes have been involved in translocation events occurring in certain leukemias and lymphomas. Furthermore a number of studies indicate that modulation of homeobox gene expression may induce alterations in proliferative, differentiative or phenotypic characteristics of hematopoietic cells. Although the function of each individual gene has not been clearly defined there is strong evidence for cooperativity among homeoproteins indicating that regulatory combinations of homeobox genes may play a pivotal role in controlling survival, proliferation and differentiation of hematopoietic cells.