Profound changes in patterns of gene expression can result from relatively small changes in the concentrations of sequence specific transcription factors. Synergistic interaction between factors bound to different sites within a transcriptional control region is supported by the work of Courey et al. (1989). Sp1 is a sequence specific transcription factor that recognizes GGGGCGGGGC and closely related sequences, which are often referred to as GC boxes. Sp1 binds to GC box promoters elements and selectively activates mRNA synthesis from genes that contain functional recognition sites. SP1 can interact with G/C rich motifs from serotonin receptor promoter. Kadonaga et al. (1987) cloned the human Sp1 cDNA and showed that it has contiguous zinc finger motifs and requires zinc for sequence specific binding to DNA.