The sera of certain patients with systemic lupus erythematosus contain an antibody-like substance capable of reacting with highly purified DNA preparations from widely divergent sources. Precipitin reactions have been demonstrated by double diffusion in agar and quantitative precipitin curves have been obtained. Complement was observed to be fixed in the reaction.

Evidence was obtained that the serum factor possessed antigenic properties similar to those of γ-globulins and migrated with this fraction on zone electrophoresis. The interaction of this factor with DNA exhibited certain specific characteristics which differ considerably from non-specific reactions between DNA and proteins in general.

The DNA-precipitating factor appeared to be one of a number of related factors reacting with nuclear constituents of many different cells. It differed in certain respects from the "LE factor" which is responsible for the formation of "LE cells."

The accumulated evidence, although not yet conclusive, favors the concept that the precipitating factor represents an antibody to DNA, and that it is one of a number of autoantibodies elicited in this disease.

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