Sp1 (specificity protein 1) belongs to the 26 member strong Sp/KLF family of transcription factors. Sp1 is a well-known member of a family of transcription factors that also includes Sp2, Sp3, and Sp4, which are implicated in an ample variety of essentially biological processes and have been proven important in cell growth, differentiation, apoptosis, and carcinogenesis. Sp1 activates the transcription of many cellular genes that contain putative CG-rich Sp-binding sites in their promoters. Sp1 also has a role in the DNA damage response; at low levels of DNA damage, Sp1 is phosphorylated by ATM and localizes to double-strand break sites where it facilitates DNA double-strand break repair. Depletion of Sp1 increases the sensitivity of cells to DNA damage, whereas overexpression of Sp1 can drive cells into apoptosis. In response to a variety of stimuli, Sp1 can be regulated through proteolytic cleavage by caspases and/or degradation.