Viruses are obligate intracellular parasites, which means that they must enter a cell to reproduce. As virions are too large to diffuse passively across the plasma membrane, cellular pathways for uptake of extracellular materials provide entry routes. The first step in entry is adherence of virus particles to the membrane, an interaction mediated by binding to one or more receptor molecules on the cell surface. Identification of cell receptors for viruses is an important objective because their study may lead to information about how the virus enters the cell, how it is targeted to specific tissues, and how it causes disease. The cell receptor for the recently identified coronavirus-EMC, which has so far infected 15 humans with 9 deaths, has been identified as dipeptidyl peptidase 4 (DPP4).