1-Acetoxy-2,5-hexanedione-13C4 is an intermediate in the synthesis of N-Acetyl-S-(3,4-dihydroxybutyl)-L-cysteine-13C4 (A173713). N-Acetyl-S-(3,4-dihydroxybutyl)-L-cysteine-13C4 is labelled N-Acetyl-S-(3,4-dihydroxybutyl)-L-cysteine (A173710), which is a metabolite of butadiene.