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:16227571). Greatly enhances MSL2 E3 ubiquitin ligase activity, promoting monoubiquitination of histone H2B at 'Lys-34' (H2BK34Ub) (PubMed:21726816). This modification in turn stimulates histone H3 methylation at 'Lys-4' (H3K4me) and 'Lys-79' (H3K79me) and leads to gene activation, including that of HOXA9 and MEIS1 (PubMed:21726816). In the MSL complex, acts as a scaffold to tether MSL3 and KAT8 together for enzymatic activity regulation (PubMed:22547026). Component of a multisubunit histone acetyltransferase complex (MSL) at least composed of the KAT8/MOF/MYST1, MSL1/hampin, MSL2 and MSL3 (PubMed:16227571, PubMed:16543150). Forms a MSL heterotetrameric core with MSL2 (PubMed:16227571). Interacts with KAT8 and MSL3; both interactions are direct (PubMed:22547026, PubMed:21217699, PubMed:30224647). Directly interacts with NUPR1 (PubMed:19650074). Interacts with TP53BP1; this interaction may be required for MSL1 DNA repair activity, but not for histone acetyltransferase activity (PubMed:19650074). Interacts with TTC4, ECM2 and PIHD1 (By similarity). Up-regulated by gamma-irradiation. The coiled coil is formed by helices from two subunits in the MSL1 homodimer. Sumoylated with SUMO1. Belongs to the msl-1 family.