We ask questions such as “What is the structure of F‐actin”. Since many proteins polymerize into helical filaments, answering these questions is essential to understanding the functional forms of these proteins. Unfortunately, high resolution structural information is available for very few polymers. One of the main reasons for this lack of structural information has to do with the variability of these polymers. Over the past 7 years, we have developed a new approach for reconstructing helical filaments from electron microscopic images. This method surmounts many of the problems present when using traditional approaches, and has now been applied to systems as diverse as the myosin thick filament, filamentous bacteriophage, pili, bacterial actin homologs and recombination filaments.I will show, using examples from a number of polymers, that these filaments can exist in a multiplicity of states. While we have described this phenomenon for F‐actin in the past, we can now show that it applies also to RecA‐DNA filaments or Rad51‐DNA filaments, or even bacterial Type III secretion system needles. Thus, for many of these polymers there is no single structure. Rather, explicating the function of these polymers requires understanding the multiplicity of states that can exist.

Egelman, E. H. (2008). Polymorphic Perversity in Protein Polymers. The FASEB Journal, 22(S1). Portico.