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Cao Teng served as an Attendant at the Yellow Gates () in the Han imperial court during the reign of Emperor An. In 120, when Emperor Shun was still a prince, Empress Dowager Deng appointed Cao Teng to serve the prince because Cao Teng was honest, modest and prudent. Cao Teng gained the trust of Emperor Shun and became his personal friend. In 126, after Emperor Shun ascended the throne, Cao Teng was promoted to Central Regular Attendant (), the highest possible rank for a court eunuch.