HBB is the domain on DEAD-box eukaryotic DNA repair helicases (EC:3.6.1.-) that appears to be a unique fold. It's conformation is of alpha-helices 12-16 plus a short beta-bridge to the FeS-cluster domain at the N-terminal. The full-length XPD protein verifies the presence of damage to DNA and allows DNA repair to proceed. XPD is an assembly of several domains to form a doughnut-shaped molecule that is able to separate two DNA strands and scan the DNA for damage. HBB helps to form the overall DNA-clamping architecture [1]. This family represents a conserved region within a number of eukaryotic DNA repair helicases (EC:3.6.1.-).

1: Crystal structure of the FeS cluster-containing nucleotide excision repair helicase XPD. Wolski SC, Kuper J, Hanzelmann P, Truglio JJ, Croteau DL, Van Houten B, Kisker C; PLoS Biol. 2008;6:e149. PMID:18578568