This book contains everything to know about Epsin, an endocytic protein. What kind of protein is it? How does it interact with others? What kind of function can it perform? What is its role during development? How is it involved in Notch signaling? Epsin is an endocytic protein that binds Clathrin, the plasma membrane, Ubiquitin, and also a variety of other endocytic proteins through well-characterized motifs. Although Epsin is a general endocytic factor, genetic analysis in Drosophila and mice revealed that Epsin is essential specifically for internalization of ubiquitinated transmembrane ligands of the Notch receptor, a process required for Notch activation. By using Drosophila as a model, the author tested each of the five protein or lipid interaction modules of Epsin for a role in Notch activation.