A series of 1,4-naphthoquinone derivatives (1-11) were synthesized as by a previously described method (Prachayasittikul et al., 2014). A recombinant human aromatase (CYP19) and O-benzyl fluorescein benzyl ester (DBF) were supplied with the BD GentestTM kit from BD Biosciences-Discovery Labware (Woburn, USA). Dimethyl sulfoxide (DMSO) was obtained from EMD Millipore (Billerica, MA, USA).