Receptors and entry cofactors for retroviruses include single and multipletransmembrane-spanning proteins as well as newly described glycophosphatidylinositol-anchored and secreted proteins

In the past few years, many retrovirus receptors, coreceptors, and cofactor s have been identified. These molecules are important for some aspects of v iral entry, although in some cases it remains to be determined whether they are required for binding or postbinding stages in entry, such as fusion. T here are certain common features to the molecules that many retroviruses us e to gain entry into the cell. For example, the receptors for most mammalia n oncoretroviruses are multiple membrane-spanning transport proteins. Howev er, avian retroviruses use single-pass membrane proteins, and a sheep retro virus uses a glycosylphosphatidylinositol-anchored molecule as its receptor . For some retroviruses, particularly the lentiviruses, two cell surface mo lecules are required for efficient entry. More recently, a soluble protein that is required for viral entry has been identified for a feline oncoretro virus. In this review, we will focus on the various strategies used by mamm alian retroviruses to gain entry into the cell. The choice of receptors wil l also be discussed in light of pressures that drive viral evolution and pe rsistence.