C4A and autoimmune disease: The HERV-K(C4) prototype sequence is, in fact, normally expressed in various human cells, almost exclusively producing antisense transcripts [18, 21] that have been hypothesize to act as i) regulators of homologous genes expression through antisense inhibition, ii) possible defense mechanism against exogenous infections, iii) potential contributor to autoimmune diseases involving the complement components [21].