Since the late 1980s, in France and in a number of other countries, cancer genetics testing has become a clinical reality, particularly for hereditary breast and ovarian cancer. BRCA tests allowing for the assessment of an increased cancer risk among patients and their healthy relatives are now being routinely performed as part of clinical practice. Based on fieldwork on French clinical cancer genetics and on the French Cancer Genetics Collaborative Network, this paper examines the configuration of entities, actors and activities mobilized by the performance of BRAC testing, and argues that the development of clinical molecular genetic practices is predicated upon the development of new forms of collaborative work that lead to a transformation of the content and organization of medical activities and judgements. The paper analyses three major collective configurations - local multidisciplinary collectives, data collectives and new clinical collectives - and argues that they not only provide the material conditions needed to carry out the relevant activities, but also articulate a series of distinctive bio-clinical interventions. These interventions provide an interface with research activities, produce the epidemiological measurements and tools that are a sine qua non for clinical work in this field, and, most importantly, establish the conventions that underlie practices, which define the criteria that turn tools and novel entities into operational components of clinical settings. It thus appears that in the field of clinical cancer genetics, bioclinical collectives, as a locus of expertise, have replaced the individual judgement of the practicing clinician.