The HirA B (Histone regulatory homolog A binding) motif is the essential binding interface between HIRA pfam07569 and ASF1a, of approx. 40 residues. It forms an antiparallel beta-hairpin that binds perpendicular to the strands of the beta-sandwich of ASF1a N-terminal core domain, via beta-sheet, salt bridge and van der Waals interactions. The two histone chaperone proteins, HIRA and ASF1a, form a heterodimer with histones H3 and H4. HIRA is the human orthologue of Hir proteins known to silence histone gene expression and create transcriptionally silent heterochromatin in yeast, flies, plants and humans. The yeast CAF1B proteins which bind H3 also carry this motif at their very C-terminus.