This paper examines the utility of using concepts from the sociology of technology to investigate how new technologies and new disease concepts are co-constructed. For researchers to introduce gene therapy into experimental clinical practice, they had to engage in a process of heterogeneous social-technical engineering. This included: the construction of local networks of regulators, genes, firms, clinicians and patients; the social shaping of gene therapy technology as a novel form of drug delivery; the creation of a new industry; and the reconceptualisation of many common acquired diseases as being genetic in some way. These changes marked a shift from an account of genetic disease based on the inheritance of deleterious genes, to one which explained acquired conditions in terms of a 'molecular pathology', resulting from errors in the way genes are regulated. This process of socio-technical change has resulted in the construction of a new type of 'genetic body'.