EFS (embryonal Fyn-associated substrate) is a docking protein that serves as a central coordinator in tyrosine-kinase-based signaling related to cell adhesion. As a member of the CAS (Crk-associated substrate) adaptor protein family, EFS lacks enzymatic activity but contains specific recognition and binding sites for assembly of signaling complexes essential for cell proliferation, survival, and migration 1. EFS interacts with the SH3 domain of FYN and associates with SRC family kinases at focal adhesions, where it functions as an intermediate in integrin-dependent signaling pathways that activate downstream effectors regulating the actin cytoskeleton 1. The protein localizes to focal adhesions and the plasma membrane, participating in cell surface receptor protein tyrosine kinase signaling and intracellular signal transduction. While EFS and its family member CASS4 have been less extensively studied than other CAS proteins, recent evidence implicates EFS in immune system function and the pathogenesis of various diseases including developmental disorders, autoimmune conditions such as Crohn's disease, Alzheimer's disease, and cancer 1. Structure-function analyses indicate EFS conserves many functional elements with better-characterized CAS family members, suggesting it plays important but incompletely understood roles in cellular signaling and disease pathogenesis.