Sp transcription factor family has a highly conserved DNA-binding domain composed of zinc fingers. The Sp family is comprised of Sp1, Sp2, Sp3 and Sp4 that regulates transcription by binding to consensus GC- and GT-box regulatory elements in the target genes. Sp1, Sp3 and Sp4 have a high binding capacity for GC box while Sp2 binds weakly to the GT box. Sp3 has been shown to have to a bifunctional transcription factor that can either stimulates or represses the transcription of various genes.