Regression analysis determined that C4 and HERV-K CNs, sex, age, and SSc explained about 12% of C4 serum concentration variance (P = 1.4 × 10−20) with HERV-K copies again weakening C4 levels (bHERV-K = −0.03, P = 9 × 10−7) (Fig. 2b and Supplementary Table 5). This evidence concerns the gene C4A and systemic sclerosis.