The invention relates to compounds of formula 1 wherein R1-R2 is a guanine derivative; X is oxygen or sulfur; R3 is a heteroaromatic, unsaturated heterocyclyl or an alkynyl group with the double or triple bond connected to CH2; R4 is a linker; and L is a label, a plurality of same or different labels, a bond connecting R4 to R1 forming a cyclic substrate, or a further group —R3—CH2—X—R1-R2. The invention relates also to methods of manufacture of such novel AGT substrates, to intermediates useful in the synthesis of such AGT substrates, and to a method of transferring a label from such AGT substrates to O6-alkylguanine-DNA alkyltransferase (AGT) fusion proteins comprising proteins of interest.