Transcription factor, illustration. Transcription factors contain a DNA (deoxyribonucleic acid)-binding domain and a regulatory domain, which will either upregulate or repress transcription. There are many families of transcription factors, classified by the type of DNA-binding domain they contain, such as the homeodomain or zinc-finger domain. Most are nuclear proteins, although some exist in the cytoplasm until they are activated and then enter the nucleus.