Canonical histone H3 and histone variant H3.3 are posttranslationally modified with the genomic distribution of these marks denoting different features and these modifications may influence transcription. While the majority of posttranslational modifications occur on histone tails, there are defined modifications within the globular domain, such as acetylation of H3K122/H3.3K122. To understand the function of the amino acid H3.3K122 in transcriptional regulation, we attempted to generate H3.3K122A mouse embryonic stem (mES) cells but were unsuccessful. Through multi-omic profiling of mutant cell lines harboring two or three of four H3.3 targeted alleles, we have uncovered that H3.3K122A is neomorphic and results in lethality. This is surprising as prior studies demonstrate H3.3-null mES cells are viable and pluripotent but exhibit a reduced differentiation capacity. Together, these studies have uncovered a novel dependence of a globular domain residue within H3.3 for viability and broadened our understanding of how histone variants contribute to transcription regulation and pluripotency in mES cells.