Darden’s construction strategies are strategies for generating new hypotheses about a mechanism. In addition to decomposition and localization, Darden shows that scientists often borrow a schema type from another area of science, as when selection-type mechanisms were borrowed to understand how the immune system works, or assemble a mechanism from known modules of functional activity (modular sub-assembly), as is common in biochemistry and molecular biology. Sometimes, scientists know one part of the mechanism and attempt to work forward or backward through to the other parts and activities. In the discovery of the mechanism of protein synthesis, for example, molecular biologists worked forward from the structure of DNA to figure out what molecules could interact with it (forward chaining), and biochemists worked backward from proteins to figure out what chemical reactions would be necessary to create them (backward chaining). They met in the middle at RNA. Protein synthesis is now understood to involve transcribing DNA into RNA and then translating RNA into proteins. Far from being philosophically inscrutable, Darden points out that scientists used what they knew about the working entities and activities in the mechanism to infer what could come next or before in the mechanism of protein synthesis (Darden 2006; see also the entry on  molecular biology).