18I IN BIOSYNTHETIC [131I]THYROXINE 119 of iodine. Coupling in the 3:5 positions occurs only slightly or not at all. 3. This reaction has been utilized as the basis of a method for the localization of 131I in the thyroxine molecule. 4. The method is applicable to biosynthesized [131I]thyroxine. I am indebted to the Rockefeller Trust of the Medical School and to the Medical Research Council for facilities for carrying out this work. I wish to thank Miss Diana Walmsley for technical assistance.

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A remarkable difference has been observed between the reactivity of the two forms of human complement component C4. C4B binds twice as effectively as C4A to antibody‐coated red cells, but the reverse occurs with protein‐antigen complexes. C4B reacts much more effectively with hydroxyl groups than C4A and this is reversed for reaction with amino groups in spite of the very small difference in amino acid sequence between the two forms of C4. No other differences in stability, activation or inactivation were observed. These findings emphasise the biological advantage of the duplication of the C4 gene in its reaction with a wide range of antigenic structures. The correlation of the presence of different forms of C4 with susceptibility to autoimmune diseases may be explicable by these big differences in binding reactivity.

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Four human complement genes, which have previously been mapped between HLA-D and HLA-B on chromosome 6, have now been aligned on a 98-kilobase (kb) section of the chromosome on the basis of four overlapping cosmid clones of genomic DNA. The C2 and factor B genes, less than 2 kb apart, are about 30 kb from two C4 genes separated from each other by about 10 kb.