In this article we examine the history of the production of microarray technologies and their role in constructing and operationalizing views of human genetic difference in contemporary genomics. Rather than the “turn to difference” emerging as a post-Human Genome Project phenomenon, interest in individual and group differences was a central, motivating concept in human genetics throughout the twentieth century. This interest was entwined with efforts to develop polymorphic “genetic markers” for studying human traits and diseases. We trace the technological, methodological (...) and conceptual strategies in the late twentieth century that established single nucleotide polymorphisms as key focal points for locating difference in the genome. By embedding SNPs in microarrays, researchers created a technology that they used to catalog and assess human genetic variation. In the process of making genetic markers and array-based technologies to track variation, scientists also made commitments to ways of describing, cataloging and “knowing” human genetic differences that refracted difference through a continental geographic lens. We show how difference came to matter in both senses of the term: difference was made salient to, and inscribed on, genetic matter, as a result of the decisions, assessments and choices of collaborative and hybrid research collectives in medical genomics research. (shrink)