The cell surface is the barrier between the cell and its environment which regulates the flow of both simple and complex molecules into and out of the cell; it is also the organelle responsible for communication between the cell and its environment. Each cell expresses receptors for a wide variety of hormones, growth factors, growth substrates and other cells. In multicellular organisms communication between cells is required for controlling development, cellular differentiation, morphogenesis and, in a more general sense, integration of myriad cell types into a single organism. The series Receptors and Recognition has as its overall aim the dissection of the cell surface to correlate structure and function for this complex organelle. In most of the preceding volumes the approach has been biochemical or physiological. In this volume the mammalian cell surface is analysed by a genetic approach. Genetic analysis of the cell surface, especially when combined with immuno­ logical techniques, has a long history. In 1900 Landsteiner showed that serum from one individual could agglutinate the red cells of another. Besides the practical result of making blood transfusion safe, this was the first demon­ stration of a human genetic polymorphism and for the next 50 years the red blood cell surface provided most of the genetic markers used to study human populations.