Abstract: The invention provides a method of selecting a mutant polypeptide having lysine demodification, in particular lysine deacylation, activity, wherein the method comprises the following steps (a) incubating a mutant polypeptide having an amino acid sequence with at least 80% sequence identity to SEQ ID NO: 1 with a peptide or polypeptide comprising an inactivated essential lysine residue; and (b) determining the activity of the mutant polypeptide to activate the peptide or polypeptide comprising the inactivated essential lysine residue, wherein the mutant polypeptide and the peptide or polypeptide comprising an inactivated essential lysine residue are incubated in a biological cell. The invention furthermore relates to an acylated luciferase, particularly Firefly luciferase, and uses thereof.