The last stage of pheomelanogenesis is the oxidative polymerization of BT, BTCA, and the products of secondary modifications of the benzothiazine moieties of these to form pheomelanin. Several dimeric and trimeric intermediates have been identified (Napolitano et al. 2001) but it is unclear whether these are major components of natural pheomelanin pigments. Most studies have used powerful chemical oxidants (Di Donato et al. 2002, Napolitano et al. 2008). which may lead to a pheomelanogenesis process that differs from the in vivo process (Wakamatsu et al. 2008).