Introduction 3. Techniques Used for Measuring Binding Affinity 4. Organization of the Erythrocyte Membrane Skeleton 4.1. Maintenance of Erythrocyte Shape 4.2. Maintenance of Erythrocyte Membrane Mechanical Properties 5. 4.1 R as a Modulator in a Complex Membrane System 5.1. Facilitation of p55 Binding to GPC 5.2. Modulation of Ankyrin Binding to Transmembrane Proteins 5.3. Facilitation of Spectrin/Actin Interaction 6. Ca2+/CaM-Dependent Regulation of 4.1 R Interactions 6.1. Variations in Intracellular Ca2+ Level 6.2. 4.1 R, a CaM-Binding Protein with Unique Features 6.3. Ca2+/CaM-Dependent Regulation of Membrane Protein Interactions 7. Functional Structure of 4.1 R