Scaffold subunit of some HBO1 complexes, which have a histone H4 acetyltransferase activity. Plays a key role in HBO1 complex by directing KAT7/HBO1 specificity towards histone H4 acetylation (H4K5ac, H4K8ac and H4K12ac, regulating DNA replication initiation, regulating DNA replication initiation. May also promote acetylation of nucleosomal histone H4 by KAT5. Promotes apoptosis. May act as a renal tumor suppressor. Negatively regulates canonical Wnt signaling; at least in part, cooperates with NPHP4 in this function.