The present invention relates to fusion proteins which are capable of binding to phosphatidylserine comprising a phosphatidylserine binding ligand and a modified O6 -alkylguanine-DNA alkyltransferase which is capable of autoconjugation to an O6 -benzylguanine-modified label, the fusion proteins being capable of binding to phosphatidylserine on the surface of a cell undergoing apoptosis. The invention also relates to recombinant polypeptide precursors of the fusion proteins which comprise a secretion leader sequence, purification tag, protease cleavage site and the fusion protein. Also included in the scope of the invention are nucleic acids encoding the recombinant polypeptide precursor, vectors comprising the nucleic acids, host cells comprising the vectors, methods of production of the fusion proteins, kits and assays for detecting apoptosis.