Aurones belong to a class of plant flavonoids that provides the bright yellow color of some important ornamental flowers, such as snapdragon (Antirrhinum majus). Despite the potential significance of the enzyme(s) and gene(s) responsible for aurone biosynthesis in biotechnology, the enzymological details of aurone biosynthesis were not established until recently. During the past two years, however, the biosynthetic pathway of aurones in yellow snapdragon has been elucidated, and a key enzyme of the pathway, aureusidin synthase, and its cDNA have been isolated. This enzyme was identified as a chalcone-specific homolog of plant polyphenol oxidase and this provides unambiguous evidence for a new biological function of plant polyphenol oxidases, i.e., flower coloration.