Antibody Validation 01: Antibody Specificity: What's the problem?

In this introduction to the topic, Professor Andreas Plückthun, University of Zürich, discusses the biology of the 3 principal types of commercial antibodies: polyclonal, monoclonal, and recombinant. Surprisingly, only recombinants offer a reasonable hope of reproducibility in the long term - yet they remain very much the minority.

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I have a question about the antibody basics, where does the number 10^12 comes from. XXX sites multipled by 21 ?