The invention relates to new proteins called alkylcytosine transferases (ACTs) derived from O6-alkylguanine-DNA alkyltransferase, and to substrates for ACTs specifically transferring a label to these ACTs and to fusion proteins comprising these. The substrates according of the invention are substituted cytosines of formula (I) wherein R1 is an aromatic or a heteroaromatic group, or an optionally substituted unsaturated alkyl, cycloalkyl or heterocyclyl group with the double bond connected to OCH2-; R2 is a linker; and L is a label or a plurality of same or different labels. The invention further relates to methods of transferring label L from these substrates of formula (I) to ACTs and ACT fusion proteins. The system of ACT-compound of formula (I) is particularly suitable for double labelling studies together with the known system O6-alkylguanine-DNA alkyltransferase (AGT)-benzylguanines.