Hydroquinones are known for their redox properties and play an important role in a range of biological processes , where the formation of a radical semiquinone is key to the oxidation progression from hydroquinone to semiquinone to quinone. The first observation of a reducing substance in AKU patients' urine was reported in 1859 . Radical formation can explain the negative interference observed on a range of routine clinical assays that are based on the production of hydrogen peroxide in samples with elevated HGA concentration . EPR spectra (Fig. ) of the HGA-derived radical resembled that of melanins and polyhydroquinones . Keeping in mind that a similarity in electronic structure as measured by EPR does not require high similarity in chemical structure, we reconsider the process of AKU pigment formation, with reference to melanins and (hydro/semi-)quinones as appropriate.