The human receptor for the complement cleavage fragments C3b and C4b (complement receptor type 1, CRl) is found on the surfaces of erythrocytes, most peripheral blood leukocytes, glomerular podocytes, and follicular dendritic cells, and in plasma (1-3). Besides the removal of C3b-or C4b-coated microorganisms or immune complexes (4), CRl also serves as an inhibitor of the C3 and the C5 convertases by dissociating C2b and Bb fragments and by acting as a cofactor for the factor I-mediated cleavage of C3b and C4b (5, 6) . This regulatory capacity is shared by the structurally related members of a supergene family, termed the regulator of complement activation (RCA)t region, located on chromosome 1 (7-13) .Human CRl is a single chain glycoprotein with four allotypic variants that differ in Mr on SDS-PAGE by increments of 40-50 kD (14-17). The two most common variants are termed F and S (or A and B allotypes) and exhibit Mr of 250 and 290 kD, respectively. These variations reflect differing lengths of the polypeptides and not posttranslational modifications, since distinct unglycosylated precursors have been described (18) and incremental differences of 1.3 to 1.5 kb were also observed in the CRl transcripts from various allotypes (19,20). The cDNA that encodes the entire F allotype of 2,039 amino acids (aa) has been sequenced and the extracellular portion of the molecule is found to comprise 30 short consensus repeats (SCR). A feature that distinguishes CR1 from the other proteins of this gene family is the organization of the NH2-terminal 28 SCRs into four tandem long homologous repeats (LHR) of about 450 aa, each containing seven SCRs . Extensive sequence homologies of 60 to 99% have been observed among the LHRs, suggesting that they have arisen by gene duplication (21,22).

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