Greece recognizes only a Muslim minority, and no ethnic or national minorities, such as Turks, Pomaks or Bulgarians. This is the result of several international treaties as the Convention Concerning the Exchange of Greek and Turkish Populations of 1923 and of the Treaty of Lausanne of 1924, when the old millet categories were used for the population exchanges of the Greek Orthodox Christians from Turkey (except from Istanbul, and the isles of Gökçeada and Bozcaada) and Muslims from Greece (except from Western Thrace), as well as for the protection of the two remaining recognized minorities, the "Muslims of Western Thrace" (Turks, Pomaks and Roms) and the "Greek Orthodox of Istanbul". In 1924 upon the League of Nations' demand, a bilateral Bulgarian-Greek agreement was signed, known as the Politis–Kalfov Protocol, recognizing the "Greek Slavophones" as Bulgarians and guaranteeing their protection. On 2 February 1925, the Greek parliament, claiming pressure from the Kingdom of Yugoslavia, which threatened to renounce the treaty about the Greek–Serbian Alliance of 1913, refused to ratify the agreement, that lasted until 10 June 1925. In 1927 Mollov–Kafantaris population exchange agreement was signed and the bulk of the Slavic-speaking population in Greece left for Bulgaria.