Core histone-binding subunit that may target chromatin remodeling factors, histone acetyltransferases and histone deacetylases to their histone substrates in a manner that is regulated by nucleosomal DNA. Binds directly to helix 1 of the histone fold of histone H4, a region that is not accessible when H4 is in chromatin. Belongs to the WD repeat RBAP46/RBAP48/MSI1 family.