Our results showed a sexual dimorphism with respect to the protection afforded by C4A and C4B. While in female individuals, C4A copies grant slightly more protection than C4B copies, our data suggest that in male individuals only C4A confers protection while we did not observe a strong effect for C4B. In male individuals, C4B might therefore function like a null allele with respect to protection from SSc as higher CNs of C4B are associated to higher SSc risk (Fig. 1c). The gene discussed is C4A; the disease is systemic sclerosis.