A former Greek Orthodox patriarchal basilica (church), later an imperial mosque, and now a museum. From the date of its construction in 537 until 1453, it served as an Eastern Orthodox cathedral and seat of the Patriarchate of Constantinople, except between 1204 and 1261, when it was converted to a Roman Catholic cathedral under the Latin Empire. The building was a mosque from 29 May 1453 until 1931. It was then secularized and opened as a museum on 1 February 1935. The building contains world-wide known impressive mosaics including Emperor Alexander's and Empress Zoe's.
It represents the most typical example of architecture of the Byzantine middle period in Constantinople and is, after Hagia Sophia, the second largest religious edifice built by the Byzantines in Istanbul. According to some souces, Irene of Athens, the well-known Byzantine empress regnant from 797 to 802, is also burried here.