The incidence of systemic lupus erythematosus (SLE) and systemic sclerosis (SSc) is markedly
increased in women. Both sex hormones and X chromosomes might contribute to this sex bias. The
dosage of X-linked genes is equilibrated between men and women due to the inactivation of one X
chromosome (XCI) in female cells. However, XCI is incomplete, leading to increased expression of
some X-linked genes.