HOXA9 is an animal protein encoded in the HOXA9 gene, which is important in maintaining stem cell populations and guiding their differentiation in mammalian development, where three are three distinct stages of erythrocyte formation,, with adult erythrocyte being the most common.
Homeobox protein Hox-A9 is a protein that in humans is encoded by the HOXA9 gene. In vertebrates, the genes encoding the class of transcription factors called homeobox genes are found in clusters named A, B, C, and D on four separate chromosomes. Throughout the development of a mammal, there are three distinct stages of erythrocyte formation  embryonic, fetal and adult. Adult erythrocytes are the most common blood cell type in mammals, and their characteristic biconcave shape, 7-8 m diameter and enucleation are amongst the greatest commonalities between mammalian species. Indeed in the hematopoietic lineage, it has been found that HOXA9 is preferentially expressed in hematopoietic stem cells (HSCs), and is down-regulated as the cell differentiates and matures further. Furthermore, transgenic mice with overexpressed HOXA9 developed a 15-fold increase in the amount of committed progenitor cells in the bone marrow, indicating that overexpressed HOXA9 induces expansion of the HSC population without disrupting differentiation. From these results, it appears that HOXA9 is important in maintaining HSC populations, as well as guiding their differentiation, especially towards myeloid (erythroid and granulocyte) lineages. 
