Mammalian Numb (Numb) encodes an endocytic adaptor protein first characterized in the Drosophila nervous system as an intrinsic cell fate determinant which is asymmetrically localized and preferentially segregates into only one of the two daughter cells upon division (Rhyu et al. 1994; Knoblich et al. 1995; Spana and Doe 1995). Mammalian Numb homologues have been identified, and homozygous deletion of Numb in mice leads to embryonic lethality, suggesting that Numb plays an essential role in mammalian development (Verdi et al. 1996; Zhong et al. 1996). In addition to playing a role in asymmetric cell division(ACD), Numb has been shown to function in endocytosis, ubiquitination, cell adhesion, migration, and cancer.