Although anti-SmD peptide antibodies represent only a minor subpopulation of anti-Sm antibodies, based on the high sensitivity and specificity percentages as well as the observation that anti-SmD peptide antibodies can be used to discriminate MCTD from SLE patients, we conclude that these subpopulations represent important SLE-specific antibodies [14,15]. This evidence concerns the gene SNRPN and mixed connective tissue disease.