Abstract Homeobox-containing genes encode transcription factors implicated in the development and differentiation of multiple cell systems, including hematopoiesis. Several lines of evidence, obtained in our laboratory, indicate that Otx1, a homeobox gene of the paired class strictly required for brain morphogenesis, plays an important role also in blood cell production. It is differentially expressed in hematopoietic progenitors, particularly within the erythroid lineage and loss of Otx1 function results in a cell-autonomous erythroid impairment. Furthermore, Otx1 contributes to the control of erythropoiesis through a direct action on Scl/Tal1, a major hematopoietic regulator. In this study we have investigated whether Otx1 is also implicated in the regulation of myelo-monocytic differentiation. Analysis of Otx1 expression indicated that the gene is transcriptionally active in myelo-monocytic precursors. Moreover, using clonogenic assays we observed that Otx1−/− mice display abnormal frequencies of bone marrow myeloid subpopulations, although the total number of myeloid precursors is normal, as compared to wild type animals. Inactivation of Otx1 leads to a significant decrease of myelo-monocytic (CFU-GM) and monocytic (CFU-M) progenitors, and a concomitant increase of granulocytic precursors (CFU-G). In addition, morphological analysis of Otx1−/− bone marrow cells confirmed a high percentage of cells of the granulocytic lineage. Furthermore, at the molecular level, we detected in null mutant cells the upregulation of the gene encoding the G-CSF receptor and the modulation of other key myeloid regulators. Finally, to understand whether the action of Otx1 in myeloid cells is, like in erythoid cells, mediated by Scl we tested whether expression of a Scl transgene in Otx1−/− mice can re-establish normal ratios between the myeloid subpopulations. Results showed that, in contrast to the findings within the erythroid compartment, constitutive Scl expression does not fully rescue the alterations within the myelo-monocytic lineage. Taken together our data indicate that the brain morphogenetic gene Otx1 is an important regulator of the blood system, as it plays a relevant role in erythropoiesis, and contributes to control also myelo-monocytic differentiation, likely at the level of the commitment events of bipotential progenitor cells.