The basilica of San Stefano Rotondo, St Stephen in the Round, was the first central plan church built in Rome. The Liber Pontificalis says that it was built by order of Pope Simplicio (468-483). Recent excavations under its floor have revealed its original liturgical arrangement in which the altar was not placed in the geometric centre of the central drum, but at the end of a processional way, thus off-centre within the central drum.
Santo Stephano Rotondo video
Here is a video showing the interior. See especially minutes 6:39-8:38.
San Stefano Rotondo panorama
Here is a lovely panorama view of the church found on flicker: