This paper compares the development of genetic testing for breast cancer (BRCA testing) in the USA and the UK. It argues that national political cultures played an important role in how these genetic testing technologies were shaped, and that the shapes of these technologies had important implications for the users of these systems. In order to demonstrate the roles of national social and political elements in the development of new genetic testing technologies, I introduce the concept of a technology's architecture, which is made up of component and the specific ways in which these components are assembled to fulfill particular functions. In the USA, four very different BRCA testing systems initially emerged. However, one biotechnology company, Myriad Genetics, eventually used its legal and economic position to become the sole provider of testing. It offered BRCA testing the way many other laboratory tests were provided in the USA, available to anyone through any physician. The shape of this testing service had important implications for its participants, defining the client as a consumer who could demand access to any of Myriad's laboratory services, but could not choose among testing systems. In the UK, the government-run National Health Service provided testing through regional genetics clinics, using family history information to assess risks and triage care. Clients in the UK were defined as citizens and patients, who had the right to equal access to the testing system but could not demand any specific services. [ABSTRACT FROM AUTHOR]