The Basilica of Saint Mark (San Marco) is one of the most remarkable examples of Byzantine influence in Italy. As the primer landmark of Venice, it is perhaps the most famous example outside of Istanbul. 

In 828, Venetian merchants stole the relics of Saint Mark the Evangelist from their original resting place in Alexandria, Egypt. It is said the Venetians hid the relics in a barrel under layers of pork to get them past Muslim guards. The adventure is depicted in the 13th-century mosaic above the door farthest left of the front entrance of the Basilica.
The relics were initially housed in a temporary chapel within the Doge's Palace, but a more substantial church was built to shelter the valuable relics in 829-32. This burned in a rebellion against Doge Pietro Candiano IV in 976, but was restored by Doge Domenico Contarini (d. 1070). The present basilica, which incorporates the earlier buildings, was completed around 1071. 
While the basic structure of the building has changed very little over the last millennium, its decoration was regularly modified after its completion. The succeeding centuries, especially the 14th, all contributed to its adornment, and Venetian vessels from the Orient brought a virtually continuous supply of columns, capitals, or friezes from ancient buildings to adorn the basilica. It was modeled after the Church of the Holy Apostles, where Fatih Mosque is now located. 
The exterior brickwork was gradually covered with various marbles and carvings, some much older than the building itself. A new frontage was constructed and the domes were covered with higher wooden domes in order to blend in with the Gothic architecture of the redesigned Doge's Palace. San Marco was the chapel of the Doges for most of its history, but in 1807, it became the cathedral of Venice.
It also has several noteworthy additions in and around in building. The most famous is the Triumphal Quadriga - four horses which were possibly located on the starting gates of the Hippodrome. The originals are now stored inside, with copies grace the exterior, where the originals were once located. There are also two other fascinating pieces coming from Constantinople. The oldest is a porphyry sculpture - the Portrait of Four Tetrarchs, when the Roman Empire was ruled by four emperors. The so-called Pilastri Acritani (Pillars of Acre) are located nearby. They came from the Church of St. Polyeuctus, the largest church in Constantinople. San Marco has a large number of columns which clearly come from many other places - part of its spoils from the Crusades. Inside there is a magnificent altarpiece, known as the Pala d'Oro (the "Golden Cloth"). It consists of a large number of Byzantine enamels which were put together in this altarpiece, which is covered in gold and jewels. In addition, there is the Treasury of San Marco, which is full of spoils taken from across the Eastern Mediterranean, including important pieces from Constantinople.