Transcription factor PU.1 is a protein that in humans is encoded by the SPI1 gene. The PU.1 transcription factor is essential for hematopoiesis and cell fate decisions. PU.1 can physically interact with a variety of regulatory factors like TFIID, GATA-2, GATA-1 and c-Jun. The protein-protein interactions between these factors can regulate PU.1-dependent cell fate decisions. PU.1 can modulate the expression of 3000 genes in hematopoietic cells including cytokines. It is expressed in monocytes, granulocytes, B and NK cells but is absent in T cells, reticulocytes and megakaryocytes. Its transcription is regulated by various mechanisms.