Updated project metadata. NAA10-mediated N-terminal acetylation is widespread and has been considered essential for viability in many model organisms. However, a Naa10 null mouse model is viable and has intact global N-terminal acetylation levels. The purpose of this project was to assess the N-terminal acetyltransferase activity of NAA15 immunoprecipitates from Naa10-KO mouse liver. We show that there is a novel paralog of Naa10 present in mice, termed Naa12, with a NatA-type enzymatic activity. We demonstrate the presence of Naa12 by detecting unique Naa12 peptides in NAA15 immunoprecipitates.