Familial aspects of mixed connective tissue disease (MCTD). I. Occurrence of systemic lupus erythematosus in another member in two families and aggregation of MCTD in another family.

Three families are reported in which one member had MCTD and another member had either MCTD or systemic lupus erythematosus (SLE). In one family both mother and daughter probably had MCTD. The daughter, however, had prominent SLE features and kidney involvement. In another family, the mother had MCTD and the daughter had SLE; while in a third family one sister had MCTD and the other had SLE with prominent arthritis but negative anti-ribonucleoprotein (RNP) antibodies.