Moving modules drive biosynthesis Modular biosynthesis of small molecules—where enzyme units can be swapped in and out of assembly line complexes to produce desired products—is a distant goal in the lab despite a huge diversity of modular systems in nature. Part of the challenge is in understanding how modules interact and hand off intermediates. Reimer et al. determined crystal structures of portions of a nonribosomal peptide synthetase, including a full dimodule. Module positioning differed between these structures even when the same intermediate was attached to the enzyme. The authors used small-angle x-ray scattering to confirm that large conformational changes are possible during biosynthesis and handoff between modules. Science , this issue p. eaaw4388