||Sp1 was first detected in HeLa cells on the basis of its ability to activate the SV40 early promoter transcription. Subsequently it was shown to recognize and bind selectively to a GC-rich consensus sequence (GC-box: GGGCGG or CACCC) that presents in the promoter of several important cellular genes, including SV40 early, HIV-1, PDGF-B etc. Sp1 was the first transcription factor to be cloned and characterized. Analysis of structure and function has revealed that Sp1 can be separated into discrete functional domains. The DNAbinding domain consists of three zinc fingers that specifically bind to the GC-box element. Sp1 contains at least four separate transcriptional activation domains. Two of these domains are glutamine-rich, a wellcharacterized motif found in several other transcription factors. In addition to transcription, Sp1 function has been linked to cell growth, cancer, Huntington disease, and other disorders through transcriptional regulation or specific protein-protein interactions. The function of Sp1 can be regulated by phosphorylation and glycosylation. Natural Sp1 fraction was purified fromHeLacell nuclear extract by using wheat germ agglutinin affinity chromatography. Affinity purified Sp1 has been used for in vitro transcriptional activation, DNase I protection, and gel mobility shift assays. The Sp1 fraction is 80 - 90% pure and is devoid of other transcription factors.