For example, the presence of C4 null alleles has been associated with systemic lupus erythematosus (SLE) (Christiansen et al. 1983; Fielder et al. 1983), systemic sclerosis (Briggs et al. 1993), and myasthenia gravis (Franciotta et al. 2001). The gene discussed is C4A; the disease is systemic lupus erythematosus.