In order to improve the diagnosis of MCTD, we investigated autoantibodies selectively directed against the individual RNP components, namely, 70, A and C. We found no preferential clinical association for autoantibodies directed against RNP-70, A or C, irrespective of the IgG or IgM isotype, nor for IgG autoantibodies directed against the 40 kDa fragment of RNP-70. This evidence concerns the gene CD40LG and mixed connective tissue disease.