Cui Yan
It is not known when Cui Dan was born. He was from the "Lesser Branch" of the prominent Cui clan of Qinghe (清河, in modern Xingtai, Hebei), and his ancestors originally claimed ancestry from the ruling house of the Spring and Autumn period state Qi. Cui Dan's traceable ancestry included officials of Han Dynasty (including Cui Yan), Liu Song, Northern Wei, and Tang Dynasty. His grandfather Cui Ji () served as a staff member for a crown prince, while his father Cui Chui () served as a deputy chief imperial censor.