Situated on what was the ancient colonnaded corso di Porta Romana, San Nazaro was one of the four basilicas built during St Ambrose's evangelising drive, between 382 and 386. Constructed to accommodate relics of the apostles Andrew, John and Thomas, the church was given the name Basilica Apostolorum. When Ambrose brought along the remains of local martyr St Nazarus (who died in 396), the church was rededicated. You can see the saintly remains in the two altars of the choir, but their silver container is a copy; the one St Ambrose commissioned is in the treasury of the Duomo.
When it was built, the basilica stood outside the city walls in a Christian burial area established by Ambrose when still a bishop, hence the sarcophagi behind the church. The church was destroyed by fire in 1075 and rebuilt using material from the original structure, including the pilasters holding up the central dome. The basilichetta of San Lino, to the right of the altar, dates from the tenth century. The octagonal Cappella Trivulzio, designed by Bramantino - his only known architectural work - was added to the church in 1512. Reworked in the late 16th century and given a neoclassical interior in the 1830s, the basilica suffered considerable damage during World War II. Between 1946 and 1963, it was stripped of many of its post-fourth-century trappings to restore a sense of its early Christian austerity.