A comprehensive classification recognizes four superfamilies with well-defined structural homology: basic domains TFs (1), Zinc-coordinating domains TFs (2), helix-turn-helix (HTH) domains TFs (3), and beta-scaffold domains with Minor Groove Contacts TFs (4). Additionally, a fifth family of orphan TFs exists for which no superclass assignment can be done yet because of lack of structural information. Cys2Hys2 constitutes a superfamily of transcription factors with Zinc-coordinating DNA-binding domains, including Cys4 zinc finger domain containing TFs, such as the nuclear receptors for steroids and thyroid hormones, Cys2His2 zinc finger domain TFs, Cys6 cysteine-zinc cluster TFs and other zinc finger domain containing TFs.
 J.M. Vaquerizas et al. A census of human transcription factors: function, expression and evolution. Nat. Rev. Genetics 2009, 10, 252-263.
 P. Stegmaier, A.E. Kel, E. Wingender. SystematicDNA-binding domain classification of transcription factors. Genome Inform. 2004, 15, 276-286.