Claim: Guillaume de Marcillat, who was employed by the popes Julius II and Leo X in the Vatican in 1509, was in Arezzo to produce windows for the Cathedral of Arezzo and the Basilica of San Francesco by 1519.
Evidence: He was in Rome by 1509, where he was employed by the popes Julius II and Leo X in the Vatican and at Santa Maria del Popolo, where the two windows in the choir are his earliest surviving works. By 1519 he was in Arezzo, where he produced windows for the Cathedral of Arezzo and the Basilica of San Francesco.