Here we report an analysis of the subunit composition and substrate specificity of the NSL complex. Proteomic analyses of complexes purified through multiple candidate subunits reveal that NSL is composed of nine subunits. Two of its subunits, WD repeat domain 5 (WDR5) and host cell factor 1 (HCF1), are shared with members of the MLL/SET family of histone H3 lysine 4 (H3K4) methyltransferase complexes, and a third subunit, MCRS1, is shared with the human INO80 chromatin-remodeling complex. In addition, we show that assembly of the MOF HAT into MSL or NSL complexes controls its substrate specificity. Although MSL-associated MOF acetylates nucleosomal histone H4 almost exclusively on lysine 16, NSLassociated MOF exhibits a relaxed specificity and also acetylates nucleosomal histone H4 on lysines 5 and 8.In eukaryotic cells, chromosomal DNA is packaged with histones and other proteins into chromatin. Alterations in chromatin structure affect the accessibility of chromosomal DNA to enzymes involved in transcription, replication, and repair. Changes in chromatin structure are regulated in at least three different ways: by ATP-dependent remodeling of nucleosomes, by the incorporation of variants of histones H2A and H3 into nucleosomes, and by post-translational modifications of histones (1-5).Post-translational modifications of histones include acetylation, methylation, phosphorylation, ubiquitination, sumoylation, and ADP-ribosylation (4 -6). Reversible histone acetylation, controlled by histone acetyltransferases (HATs) 4 and histone deacetylases, plays an important role in regulation of chromatin structure and function (3, 7). Based on the nature of their catalytic domains, HATs can be grouped into two distinct families: the GCN5-related N-acetyltransferase (GNAT) family, which includes GCN5 and p300/CBP-associating factor (8), and the Moz-Ybf2/Sas3-Sas2-Tip60 (MYST) family, which is characterized by a highly conserved MYST domain composed of an acetyl-CoA binding motif and a zinc finger (9, 10). Some MYST family members also have additional structural features such as chromodomains (MOF, Esa1, and Tip60), plant homeodomain-linked zinc fingers (Moz and MORF), and other domains that bind specifically to modified histones or participate in other protein-protein interactions (10).Human MOF is an ortholog of the Drosophila MOF HAT. MOF is one of the key components of the dosage compensation or male-specific lethal (MSL) complex. The Drosophila MSL complex is composed of at least five proteins (MSL1, MSL2, MSL3, MLE, and MOF) and two non-coding RNAs (roX1 and roX2). Human cells express an evolutionarily conserved MSL complex that is composed of MOF and at least three additional subunits, including orthologs of MSL1, MSL2, and MSL3. The MOF HAT is believed to be responsible for the majority of histone H4 acetylation at lysine 16 in both Drosophila and human cells (4,(11)(12)(13).Results of recent studies suggest the existence of additional MOF-containing HAT complexes. Roeder and co-workers (14) reported that in addition...