Diagram of the modular nature of transcription factors. Factors may contain more than one activation domains, but rarely contain more than one DNA-binding domain. Gal4 and Gcn4 are yeast transcription activators. The glucocorticoid receptor (GR), which also contains a hormone-binding domain (not shown), activates transcription of target genes in the presence of certain hormones. Sp1 binds to regulatory elements in the SV40 tumor virus genome.