Kcr is a newly discovered PTM that exists in several types of eukaryotic genomes. Histone Kcr may exert charge-based cis-effects on chromatin fibers by neutralizing the positive charge of the ɛ-amino group of K. Furthermore, the increase in bulkiness and rigidity of crotonyl may lead to an enhanced effect. Therefore, histone Kcr is specifically marked as either an active promoter or potential enhancer in both human somatic and mouse male germ cell genomes.