C4B and systemic lupus erythematosus: C4 gene copy number varies from two to six for total C4, zero to five for C4A, and zero to four for C4B. Compared with healthy subjects, patients with SLE clearly have lower copy numbers of C4 and C4A, and SLE susceptibility is significantly increased among subjects with only two copies of total C4 but decreased in those with more than five copies of C4 [68].