Nectin-4 (gene name PVRL4, poliovirus receptor-like 4) is a 66 kDa type I transmembrane glycoprotein belonging to the Nectin family of Ig superfamily proteins (1). The Latin word necto means “to connect”, indicating the role of nectins in Ca²⁺‑independent cell-cell adhesion (2). Nectin-4 forms homodimers in cis, followed by interactions in trans with Nectin-1 or -4 (1-3). Human Nectin-4 mRNA is normally expressed in the placenta, especially in endothelial cells, while in the mouse it is found in the embryo, lung, testis and brain (1, 4, 5). Human Nectin-4 cDNA encodes 510 amino acids (aa), including a 31 aa signal sequence, a 318 aa extracellular domain (ECD), a 21 aa transmembrane segment (TM), and a 140 aa cytoplasmic region. Nectin ECDs contain three Ig-like domains: an N‑terminal V-type that mediates ligand binding, and two C2-type (1, 3). One Nectin‑4 isoform lacks aa 412-436 in the cytoplasmic domain (1). In many human ductal breast or non-small cell lung carcinomas, Nectin‑4 is upregulated and a soluble 43 kDa form is found in the plasma (4-6). This form is generated from the membrane protein via the action of TACE/ADAM-17 (6). The extracellular domain of human Nectin-4 shares 91%, 92%, 93%, 91% and 90% amino acid sequence homology with the corresponding regions of mouse, rat, canine, porcine and bovine Nectin-4, respectively. In forming adherens junctions, trans interactions of Nectin-4 initiate cell-cell interactions and recruit intracellular cadherins through afadin and other junctional proteins (1, 2). These interactions organize the actin cytoskeleton, strengthen attachment to basement membrane and promote further cell-cell connections (2, 7). In humans, mutation of Nectin-4 has been correlated with ectodermal dysplasia-syndactyly syndrome, indicating a role for Nectin-4 in human development (7).