Background Homeobox gene associated regulatory networks are among the key determinants of early hematopoietic development. Previously, the hematopoietic PBX interacting protein (HPIP) has been identified as a novel interacting partner of the TALE homeodomain protein PBX1, forming a microtubule signalling complex. Expression of HPIP has been associated with increased tumorigenicity of the MCF7 breast cancer cell line. We now demonstrate that HPIP is a novel regulatory protein in human hematopoiesis: constitutive expression of HPIP in human umbilical cord blood derived CD34+ cells increased the absolute number of clonogenic progenitors in liquid expansion culture as well as in methylcellulose assays with a significantly enhanced formation of erythroid colonies compared to the control (p0.01, n=6). Limiting dilution LTC-IC assays confirmed the hematopoietic activity of the protein on primitive human progenitor cells with an over 5fold increase in the absolute number of LTC-ICs compared to non-transduced cells (n=8; p<0.05). In vivo HPIP expression induced a significant shift towards myeloid engraftment (n=8;p<0.05) and doubled the proportion of hCD34+CD38+ human cells in transplanted mice (p0.05, n=8). Structure function analyses identified the C - terminal nuclear receptor/PBX interacting domain (NRPID; LXXLL domain) as a critical domain for the hematopoietic activity of HPIP. Gene expression data by microarray and Q-RT-PCR analysis demonstrated that HPIP induced particularly differential expression of genes involved in the MAPK pathway and cytokine-cytokine interaction. Taken together, these data demonstrate that proteins involved in the organization of microtubular signalling complexes such as HPIP can act as regulators of early human hematopoiesis.