Specificity of the human M-CSF receptor promoter is mediated by combinatorial activity of factors in myeloid cells. The activity and specificity of the human M-CSF receptor promoter in transient transfection studies is mediated by the 50-bp region lying between bp −88 and −40 relative to the major transcription start site in monocytic cells.52,117,215 Shown are the binding sites for C/EBP, AML1, and PU.1, all of which are important for M-CSF receptor promoter activity. In the hematopoietic system, C/EBPα is myeloid specific, AML1 is expressed in all white blood cells, and PU.1 is B-cell– and myeloid-specific, so therefore the major cell type in which all three are expressed is myeloid cells. In addition, C/EBPα and AML1 can physically interact and synergize to activate this promoter.215 Others have shown using other promoters that C/EBP proteins can synergize with PU.1.88 Also shown is CBFβ, which forms a heterodimer with AML1 and augments its ability to bind to DNA.