Core histone-binding subunit that may target chromatin assembly factors, chromatin remodeling factors and histone deacetylases to their histone substrates in a manner that is regulated by nucleosomal DNA. Component of several complexes which regulate chromatin metabolism such as the chromatin assembly factor 1 (CAF-1) complex, which is required for chromatin assembly following DNA replication and DNA repair (By similarity). Component of the chromatin assembly factor 1 (CAF-1) complex, composed of FSM (FAS1), FAS2 and MSI1. Belongs to the WD repeat RBAP46/RBAP48/MSI1 family.