Being bullied leaves a biochemical footprint in the brain. Protein lactylation is a very recently discovered post-translational modification of protein that involves the addition of a lactyl group to lysine residues. Researchers at the Institute for Comprehensive Medical Science at Fujita Health University in Japan, along with colleagues from Ibaraki University and the Japanese National Institute of Advanced Industrial Science and Technology, have now revealed in mice that protein lactylation occurs in neurons in the brain and is positively correlated with lactate levels. This modification was enhanced by neural excitation and social stress, processes known to increase lactate. The stress, from exposure to aggression, specifically enhanced lysine lactylation of histone H1 proteins. The study was published on October 12th in the journal Cell Reports.