Hagia Sophia was a Greek Orthodox Christian patriarchal basilica, later converted into an Ottoman mosque, and now a museum (Ayasofya Müzesi) in Istanbul, Turkey. From the date of its construction in 537 AD, and until 1453, it served as an Orthodox cathedral and seat of the Patriarch of Constantinople, except between 1204 and 1261, when it was converted by the Fourth Crusaders to a Roman Catholic cathedral under the Latin Empire of Constantinople. The building was a mosque from 29 May 1453 until 1931. It was then secularized and opened as a museum on 1 February 1935.