Transcription factor PU.1 (SPI1) is an 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. SPI1 can also regulate alternative splicing of target genes. Multiple transcript variants encoding different isoforms have been found for SPI1.