St Catherine was welcomed into the Dominican fold within this imposing church, and its Cappella di Santa Caterina is adorned with frescoes by Il Sodoma depicting events in her life. Catherine died in Rome, where most of her body is preserved, but her head was returned to Siena (it's in a 15th-century tabernacle above the altar in the cappella), as was her desiccated thumb (in a small window box to the right of the chapel). Next to the thumb is a nasty-looking chain that the saint flagellated herself with.