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The Byzantine Emperor Constantine XI had sent a request to the pope, asking that efforts be made to effect a union between the Latin and 'Greek' (Orthodox) Churches: for this purpose Leonard was selected to accompany Isidore, Cardinal Bishop of Sabine, to Constantinople.
# John Kempe, archbishop of York – cardinal-priest of S. Balbina, then cardinal-bishop of Santa Rufina (28 July 1452), † 22 March 1454
# Lorenzo Campeggio, bishop-elect of Feltre – cardinal-priest of S. Tommaso in Parione (received the title on 24 January 1518), then cardinal-priest of S. Anastasia (1520?), cardinal-priest of S. Maria in Trastevere (27 April 1528), cardinal-bishop of Albano (5 September 1534), cardinal-bishop of Palestrina (26 February 1535), cardinal-bishop of Sabina (28 November 1537), † 19 July 1539