was Sicily's main feudal title. In Butera, 12–13% of the population carries the sickle-cell trait, and its prevalence amongst the town's citizens is believed to be due to selective pressure against malaria. While some sources claim that it was introduced by the Muslim conquerors, other scientific studies claim that it was brought by the Greeks. At present, approximately 7% of the surnames in the city (457 out of 1,732) are attributed to a possible Hellenic origin. Main sights Arab-Norman Castle: from the 9th century Cathedral: dedicated to Saint Thomas and found in the Piazza Duomo, it was built in the 16th century. Sanctuary of San Rocco: built in the 18th century Necropolis of Piano della Fiera: in use until the 6th century BC and the Hellenistic phase, its origins date to prehistoric times Porta Reale (Royal Gate): given this name because it was entered by Norman count Roger I with a troop of Lombard soldiers Piazza Dante: the city's main square, with the town hall and the Church of S. Giovanni (St John) San Francesco: oldest church in the town, it was founded by the first Norman Christians and became a church of Franciscan friars. References and notes This article contains information translated from the Italian Wikipedia article Twin cities Gevelsberg, Germany Gambellara, Italy External links Municipalities of the Province of Caltanissetta 4th-century BC establishments in Italy 6th-century BC disestablishments 1st-millennium BC disestablishments in Italy