The CD4 membrane glycoprotein was one of the first cell surface antigens to be identified using monoclonal antibodies. It was shown to have a central role in the control of the recognition of foreign proteins by T lymphocytes and later as a receptor for the human immunodeficiency virus (HIV). The analysis of the amino acid sequence of CD4 showed that the extracellular region comprised four regions with sequence similarities to immunoglobulin domains. The structure of domains 3 and 4 of CD4 has been determined by X-ray crystallography and, like domains 1 and 2 previously determined, these have typical immunoglobulin-like folds. The results are discussed with respect to the identification of other domains with immunoglobulin-like folds from amino acid sequence data, and the evolution of CD4.