This Basilica was one of the first churches to be constructed in Milan-Sant’Ambrogio was responsible for initiating the plans to build it (4th cent.) but it was completed under the direction of his successor, San Simpliciano, who is buried there.
In 398, the sacred relics of three martyrs were placed there: those of Sisinnio, Martirio and Alessandro. Since the 4th century, the church has been remodeled many times. The most extensive work was done during the 7th century.