Employing an in vitro reconstitution approach, McGinty et al. studied the mechanism of stimulation of the Dot1-catalysed histone H3 methylation at Lys79 by histone H2B ubiquitylation at Lys120. To generate nucleosome particles that carry the ubiquitylation at Lys120, they chemically connected three polypeptides-the main parts of histone H3 and ubiquitin expressed in bacteria and a branched synthetic peptide. Using the semisynthetically produced nucleosome substrates and purified Dot1 enzyme, they showed that Dot1 is directly stimulated by the ubiquitylation, thus ruling out the need for further protein factors to mediate the effect.