Basilica Apostolorum erected by St. Ambrose in the IV century and housed relics of the Apostels and thus the original claim. The walls are still original. The celling was replaced in Middle Ages by a groin vault.
See:
- Andre Grabar, L'Âge d'or de Justinien, de la mort de Théodose à l'Islam, Paris 1966
- https://en.wikipedia.org/wiki/San_Nazaro_in_Brolo