Diagram of the most common allotypic variant of CR1.

The 30 complement control protein repeats (CCPs) constitute the extramembranous portion of CR1. These modules are followed by a hydrophobic transmembrane domain (TM) and a 43–amino acid cytoplasmic tail (CYT). There are 2 distinct functional sites (site 1 in LHR-A and 2 nearly identical copies of site 2 in LHR-B and -C), each composed of 3 CCPs. Functional differences between the sites are determined by sequence differences. The first 2 CCPs in site 1 (CCP 1 and 2) are 39% different from the first 2 CCPs in site 2 (CCPs 8 and 9), as well as from CCPs 15 and 16, and they are marked by different shading. The third CCP in site 1 is nearly identical to the third CCP (10 or 17) in site 2. CCPs 3, 10, and 17 are represented by boxes with horizontal lines.