R. Nathan b. Jehiel of Rome (1035–c. 1110), Italian lexicographer, also called Ba'al he-Arukh ("the author of the Arukh") after the title of his lexicon. Few biographical details are known of him. Some state that he belonged to the De Pomis or Delli Mansi family, but the view is widespread that he actually belonged to the famous Anau (Anav) family. He was taught in his youth by his father, a paytan and the head of the yeshivah of Rome, and may as a young man have studied in Sicily under R. Mazli'ah b. Elijah ibn al-Bazak, a pupil of R. Hai Gaon. However, there is reason to believe that the scanty references to R. Mazli'ah's name in R. Nathan's work are the addenda of an earlier copyist named R. Mevorakh, some of whose marginal notes, in which he also mentions that he was R. Al-Bazak's pupil, were later incorporated in the text of the Arukh. R. Nathan also studied under R. Moses ha-Darshan of Narbonne, as well as, in the view of some scholars, under R. Moses Kalfo of Bari and R. Moses of Pavia. When his father died immediately after R. Nathan's return to Rome about 1070, he and his two brothers R. Daniel and R. Abraham succeeded him as the heads of the yeshivah of Rome. With them he wrote responsa to halakhic questions addressed to him by various scholars, among whom was a R. Solomon Yizhaki, identified by some as Rashi. Noted for his charitable acts, R. Nathan built a magnificent synagogue and a ritual bathhouse for his community. It was while serving as head of the Rome yeshivah that he wrote his classical work (which he completed in 1101) the Arukh, a lexicon of the Talmud and the Midrashim, containing all the talmudic terms in need of explanation; in the course of time various additions were made to it. At the end of the Arukh there is a poem written in particularly difficult language and therefore of somewhat obscure meaning; in it the poet, lamenting his bitter lot, tells of the death of four out of his five sons during his lifetime.