(Chemical Equation Presented) Lighting up ubiquitylation: A photolytically removable ligation auxiliary (Aux) was employed for the site-specific attachment of ubiquitin (Ub) to one of three lysine residues in a C-terminal peptide fragment of histone H2B (cH2B). Following photolysis, the ligation product was a viable substrate for the ubiquitin C-terminal hydrolase UCH-L3. 2007 Wiley-VCH Verlag GmbH & Co. KGaA.