New York: Robert Appleton Company. ↑ Richard P. McBrien, Lives of the Popes, (HarperCollins, 2000), 245. ↑ George L. Williams, Papal Genealogy: The Families and Descendants of the Popes, (McFarland Company Inc., 1998), 43. ↑ Hayez, Michel (2002). "Gregorio XI, papa". Dizionario Biografico degli Italiani.