Alleles
Four alleles are known with predicted protein molecular weights of 190 kDa, 220 kDa, 250 kDa and 280kDa are known. Multiple size variants (55kDa-220kDa) are also found among non-human primates and a partial amino-terminal duplication (CR1-like gene) that encodes the short (55kDa-70kDa) forms expressed on non human erythrocytes. These short CR1 forms, some of which are glycosylphosphatidylinositol (GPI) anchored, are expressed on erythrocytes and the 220kDa molecular weight CR1 form is expressed on monocytes. The gene including the repeats is highly conserved in primates possibly because of the ability of the repeats to bind complement. LHR-A binds preferentially to the complement component C4b: LHR-B and LHR-C bind to C3b and also, albeit with a lower affinity, to C4b. Curiously the human CR1 gene appears to have an unusual protein conformation but the significance of this finding is not clear.
Read more about this topic: Complement Receptor 1