In 1464, Pope Pius II brought a priceless relic to Siena - the relics of the right hand of St. John. And in 1482, in the left transept, construction began on the Chapel of St. John the Baptist (Cappella di San Giovanni Battista). The chapel is round in plan, presumably designed by Giovanni di Stefano.
In the depths of the chapel is a bronze statue of John the Baptist by Donatello (1457).
The marble font in the center was created by Antonio Federighi after 1484.