GC-boxes and related motifs are frequently occurring DNA-elements present in many promoters and enhancers. In contrast to other elements it was generally thought that the transcription factor Sp1 is the only factor acting through these motifs. The cloning of paralogous genes of the Sp1 factor uncovered the existence of a small protein family consisting of Sp1, Sp2, Sp3 and Sp4. All four proteins exhibit very similar structural features. They contain a highly conserved DNA-binding domain composed of three zinc fingers close the C-terminus and serine/threonine- and glutamine-rich domains in their N-terminal regions. The high degree of structural conservation between these four proteins suggested that they do exert similar functions. Molecular, genetic and biochemical analyses, however, demonstrated that Sp2, Sp3 and Sp4 are not simply functional equivalents of Sp1. Here, I will summarize and discuss recent advances which have been made towards understanding the mode of action and biological function of individual family members.