Tumour genetics is currently turning into a massive clinical approach. This paper is an enquiry into its practices as they expand beyond expert and experimental contexts and become routinised in clinical hospital settings. Studying a French university hospital, we unpack the content and everyday organization of diagnostic labour in this context. Exploring the sociotechnical frictions that arise in the process, we describe the ways in which they are collectively controlled, and stabilized through organizational fictions, that are instrumental in making tumour genetics doable in the hospital, at a large scale. We further show that the new role of external regulations in the production of clinical values for mutations has a strong impact on diagnostic work, making it possible to be performed locally without resorting to expert bioclinical collectives, and outside the professional jurisdiction of clinical geneticists. This division of labour appears as a necessary condition for the rise in clinical productivity required by a new function assigned to genetics: to guide the prescription of drugs for common diseases. This turn in the way genetics is embedded in the clinic calls for a thorough reassessment of its impacts on clinical discourses, practices and decisions.