Profound changes in patterns of gene expression can result from relatively small changes in the concentrations of sequence specific transcription factors. 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.