This chapter focuses on cancer genetics as a field in which the extension of options for genetic testing is already clearly visible and which thus allows to study the implications of this development on an empirical level. It presents the introduction of deoxyribonucleic acid (DNA) diagnosis in the clinic as a complex process of sociotechnical change in which technical innovation is going together with the development of new skills, new relations between the clinic and the laboratory, new relations between professions, new forms of demand, new guidelines. In the context of established screening programmes for familial adenomatous polyposis, the introduction of DNA diagnosis in the early 1990s was generally seen as an opportunity to improve the organization of screening. The introduction of DNA diagnosis of hereditary breast and/or ovarian cancer is linked to initiatives which will further enhance awareness of familial breast and/or ovarian cancer risks and which will also generate increasing demand for a DNA diagnostic test.