Deficiency of C4A is a genetic determinant of systemic lupus erythematosus in three ethnic groups.

Systemic lupus erythematosus (SLE) has shown associations with the major histocompatibility complex (MHC) class II DR antigens and class III complement components C2 and C4 in previous studies. The primary susceptibility locus has been difficult to identify, however, on account of linkage disequilibrium within the MHC. We have studied C4A and C4B distributions in 63 Caucasoid, 75 Chinese and 51 Japanese SLE patients. All three populations showed a statistically significant increase in C4A*Q0 (null) alleles when compared with 323 ethnically matched controls. We conclude that complete or partial deficiency of C4A is a genetic determinant of SLE common to these three ethnically distinct populations.