Schematic diagram of the most common structural variant of complement receptor 1 (CR1*1, F allele). The amino terminal (NH2) extracellular portion is composed of 30 complement control protein repeats (CCPs; vertical boxes numbered 1–30) arranged into four long homologous regions (LHRs) A-D, each composed of seven CCPs. There are two distinct functional domains each composed of three complement control protein repeats (CCPs) (vertical hatched boxes): site 1 in LHR-A (CCPs 1–3) binds mainly C4b and has convertase decay accelerating activity, and two virtually identical copies of site 2 in LHR-B (CCPs 8–10) and LHR-C (CCPs 15–17) that bind C3b and C4b, as well as PfEMP-1, and possess Factor I- cofactor activity. Functional differences in sites 1 and 2 are determined by amino acid sequence differences. Boxes with the same hatching pattern reflect near amino acid identity of the CCPs. CCP 25 (stippled box) carries the Swain-Langley (Sl) and McCoy (McC) Knops blood group antigens. The CCP repeats are followed by a transmembrane domain (TM) and a cytoplasmic tail (CYT).