This chapter examines whether and how the Biological Weapons Convention, UN Security Council Resolution 1540, and the Chemical Weapons Convention govern synthetic nucleic acids. Building on the rules of treaty interpretation, we demonstrate that synthetic nucleic acids fall within the scope of these international non-proliferation laws. We further argue that member states are obliged to adopt control measures, including with respect to synthetic nucleic acid. To this end, member states should adopt and implement synthesis screening procedures. This procedure requires synthetic nucleic acid providers to screen the sequences being ordered and the customers who are making the order. Synthesis screening as a method for preventing the proliferation of bioweapons is widely endorsed by the biosecurity community, including BWC member states. Moreover, it is compliant with BWC obligations as it enables effective control of transfers of, and access to, prohibited nucleic acids, without hampering peaceful uses. Finally, we make three recommendations regarding the implementation of synthesis screening under the BWC. First, explicit recognition that nucleic acid screening is consistent with BWC obligations. Second, integrating the topic into future BWC work through establishing a multistakeholder initiative for developing screening best practices. Third, harmonising international and domestic control measures governing synthetic nucleic acids.