It's one of the four largest basilicas founded by order of St Ambrose. Altered during the 10C and 11C, and remodelled in the 19C, it has nonetheless retained the simplicity of early Christian edifices. The façade has a fine marble doorway. The interior, where brick predominates, is solemn. Note the frescoes by Bergognone representing the Coronation of the Virgin (1515) on the vault of the apse.