The homeobox transcription factors (TFs) are ubiquitous in eukaryotes, carrying a DNA-binding homeodomain typically 60 amino acids that folds into three α-helices. Homeodomain proteins are classified into different classes based on the sequence of the homeodomain itself, as well as on the basis of additional conserved domains found in these proteins. The atypical or TALE (three-amino acid length extension) superclass of homeobox TFs shares a three-amino-acid insertion between helix 1 and 2 and plays essential roles during embryonic development by participating in interactive TF networks. The heterodimerization of TALE proteins serves as a trigger for precise execution of developmental programs. TALE-heterodimerization is mediated by class-specific homology domains located on the N-terminal side adjacent to the homeodomain. The PBC class of TALE homeodomain proteins includes the products of the vertebrate Pbx1, Pbx2, Pbx3 and Pbx4, Caenorhabditis elegans ceh-20, ceh-40 and ceh-60 and the Drosophila extradenticle (exd) genes. The sequence similarity between the PBC proteins extends downstream of the homeodomain for about 15 amino acids, and upstream of the homeodomain is a 180-amino acid domain termed PBC. The PBC domain can be further subdivided into an A and B region [1,2,3,4,5,6,7,8].

The profile we developed covers the entire PBC domain.