The cover photograph shows Rudolf Schoenheimer in his laboratory at Columbia University. Schoenheimer was one of the first, and the most influential, biochemists to use isotopes to study metabolism. Using deuterium and ¹⁵N, he and his colleagues demonstrated that fats and proteins in an organism were in a dynamic state. His classic work is presented in the JBC Classic published in this issue of JBC Online (http://www.jbc.org/cgi/content/full/277/43/e31).