SPI1 also known as transcription factor Pu.1, is a protein that in humans is encoded by the SPI1 gene. This protein have ETS-domain transcription factor that activates gene expression during myeloid and B-lymphoid cell development. The nuclear protein binds to a purine-rich sequence known as the Pu-box found near the promoters of target genes, and regulates their expression in coordination with other transcription factors and cofactors. It can also regulate alternative splicing of target genes. Multiple transcript variants encoding different isoforms have been found for this gene. Recombinant SPI1, fused to His-tag at N-terminus, was expressed in E.coli.