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Musta'li Isma'ilism (Arabic: المستعلية, romanized: al-Mustaʿliyya) is a branch of Isma'ilism named for their acceptance of al-Musta'li as the legitimate ninth Fatimid caliph and legitimate successor to his father, al-Mustansir Billah (r. 1036–1094/1095). In contrast, the Nizari—the other living branch of Ismailism, presently led by Aga Khan IV—believe the ninth caliph was al-Musta'li's elder brother, Nizar.
The Musta'li originated in Fatimid-ruled Egypt, later moved its religious center to Yemen, and gained a foothold in 11th-century Western India through missionaries.