Autoantibodies directed against ENA are useful in the diagnosis and monitoring of various systemic connective tissue diseases. Anti-Sm antibodies are disease specific and occur in approximately 30-40% of SLE patients. SS-A (Ro), SS-B (La) and RNP are not disease specific. Antibodies to RNP occur in 35-45% of SLE patients and in over 95% of patients with MCTD. Antibodies to SS-A (Ro) and SS-B (La) occur in approximately 30-40% and 10-15% of SLE patients respectively. Antibodies to SS-A (Ro) also occur in 60% of patients with subacute cutaneous LE, in almost all cases of neonatal LE, in almost all SLE patients with C2 deficiency and in about one half of patients with Sjögren’s syndrome.