MYB factors represent a family of proteins that include the conserved MYB DNA-binding domain.The first MYB gene identified was the "oncogene" v-Myb derived from the avian myeloblastosis virus.This domain generally comprises up to three imperfect repeats, each forming a helix-turn-helix structure of about 53 amino acids. Three regularly spaced tryptophan residues, which form a tryptophan cluster in the three-dimensional helix-turn-helix structure, are characteristic of a MYB repeat.In contrast to animals, plants contain a MYB-protein subfamily that is characterised by the R2R3-type MYB domain.