In Drosophila, neuronal cell fate decisions are directed by NUMB, a signaling adapter protein with two protein-protein interaction domains, namely a phosphotyrosine-binding domain and a proline-rich SH3-binding region (PRR). The mammalian NUMB homolog plays a role in the determination of cell fate during development and binds with a variety of proteins, including Eps15, LNX1 and Notch 1. NumbL (NUMB-like protein), also known as Numb-R, NBL, CAG3A, CTG3a, NUMBLIKE or TNRC23, is a 609 amino acid cytoplasmic protein that, like NUMB, is thought to play a role in cell fate. Expressed at high levels in developing brain tissue, NumbL contains one PID (phosphotyrosine interaction domain) and plays an important role in neuronal differentiation, possibly associating with Eps15 and Notch 1. In mice, deletion of the NumbL gene is associated with early embryonic death, suggesting an essential role for NumbL in early development.