The general idea of the classification scheme is to provide a hierarchical system of taxa, inspired by both the taxonomy of biological species on the one and the enzyme catalogue on the other side. In accordance with the latter, a four-level taxonomy was proposed, comprising the ranks Superclass, Class, Family and Subfamily. Subfamily is an optional category. Underneath, two more levels were defined, which represent physical entities (i.e. genes and gene products) and have been termed Genera and Species (or "molecular species", to avoid confusion with biological species). Thus, all TFs encoded by one gene have been put into taxa on the fifth level of the TF classification, whereas the different products of one gene represent the sixth level (Species), which is displayed only if there is more than isoform.

For further details and for citation, see Wingender et al., TFClass: An expandable hierarchical classification of human transcription factors. Nucleic Acids Res.2012, doi: 10.1093/nar/gks1123 (2012).