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During the 1930s the anatomist and cell biologist Florence R. Sabin (1871-1953), the first woman to be elected to membership in the National Academy of Sciences, had investigated how antibodies were formed in response to foreign substances, using a new protein-bound red dye developed by Heidelberger as a tracer antigen. In this letter Sabin congratulated Heidelberger on the "wonderful field of further experimentation your quantitative methods have opened up," and suggested that multivalent antibodies emerge from simpler, univalent ones.