Though churches in France, Syria and Germany also claim the skull of St. John the Baptist, the relic is what gives this church its name. The skull can be viewed in the first chapel to the left of the entrance. Don't miss the stained glass in this room depicting St. John's head on a plate.
The church was built to house relics from the catacombs. These are not normally on display. Instead they're housed in the confessio under the altar and listed near the front door of the church.