Spi-B belongs to the ETS family transcription containing a conserved ETS domain which specifically binds to a purine-rich DNA sequence called the PU-box (5'-GAGGAA-3'). Spi-B has two known isoforms (28 and 18 kDa) localized in nucleus and cytoplasm. Spi-B contains two transactivation domains (TAD) at the N- terminal end which may be required for transcriptional activation of the target genes. Spi-B is involved in development of plasmacytoid dendritic cells (natural interferon producing cells) and B-cell development. Spi-B acts as a lymphoid specific enhancer. Spi-B is highly expressed in plasmacytoid dendritic cells (pDCs), B-cells and stem cell populations of the liver.