Fusion core protein from the outer coat of the human respiratory syncytial virus (RSV), molecular model. RSV causes respiratory tract infections, usually with mild symptoms. This protein is responsible for facilitating the virus's entry to cells by binding to cell membranes. The protein consists of subunits of six helices tightly bundled together. In this model, the solid colours represent the secondary structure of the protein (spiral ribbons are alpha helices), and the faded tubular structure behind represents the individual atoms and bonds.