Component of histone acetyltransferase complex responsible for the majority of histone H4 acetylation at 'Lys-16' (H4K16ac) which is implicated in the formation of higher-order chromatin structure (PubMed:24913909). Greatly enhances MSL2 E3 ubiquitin ligase activity, promoting monoubiquitination of histone H2B at 'Lys-35' (H2BK34Ub) (By similarity). This modification in turn stimulates histone H3 methylation at 'Lys-5' (H3K4me) and 'Lys-80' (H3K79me) and leads to gene activation, including that of HOXA9 and MEIS1 (By similarity). In the MSL complex, acts as a scaffold to tether MSL3 and KAT8 together for enzymatic activity regulation (PubMed:21217699). Isoform 2, isoform 3, isoform 4 and isoform 5 can medite histone H4 acetylation at 'Lys-16' (H4K16ac) (PubMed:24913909). Component of a multisubunit histone acetyltransferase complex (MSL) at least composed of the KAT8/MOF/MYST1, MSL1/hampin, MSL2 and MSL3 (By similarity). Directly interacts with MSL2 via its coiled coil domain (By similarity). Directly interacts with NUPR1 (By similarity). Interacts with TP53BP1; this interaction may be required for MSL1 DNA repair activity, but not for histone acetyltransferase activity (By similarity). Forms a MSL heterotetrameric core with MSL2 (By similarity). Interacts with KAT8 and MSL3; both interactions are direct (PubMed:21217699, PubMed:17335777). Isoform 1 and isoform 3 interact with TTC4 (PubMed:17335777). Isoform 1 interacts with ECM2 and PIHD1 (PubMed:17335777). Isoform 3 and isoform 5 are testis-specific (PubMed:16119455). Isoform 1 and isoform 4 are ubiquitously expressed (PubMed:16119455). Isoform 2 is expressed at low levels in the testis and brain (PubMed:16119455). The coiled coil is formed by helices from two subunits in the MSL1 homodimer. Sumoylated with SUMO1. Belongs to the msl-1 family.