Historians of science, reflecting the disciplinary divides of their actors, have largely examined cytogenetics and molecular biology in separate contributions. In recent years, however, as the historical analysis of postwar genetics has begun to shift toward a focus on the late twentieth century, these distinctions have become increasingly impractical. This paper examines the collaborative use of cytogenetic and molecular analysis by research groups as they examined a unique chromosomal marker associated with inherited intellectual disability, known as the fragile X site. As clinicians and biologists studied the fragile X site during the early 1980s, they noted its unusual behavior under the microscope and in family pedigrees. These findings greatly complicated attempts to use the chromosomally visible fragile X site to warn female carriers and identify fetuses affected by fragile X syndrome, leading to the recruitment of molecular biologists to develop improved diagnostic techniques. Molecular biologists were also drawn to study the fragile X site by the potential basic research implications of its unusual behavior. Collaborating across international and interdisciplinary divides, cytogeneticists and molecular biologists established the fragile X site as a novel and unanticipated example of genomic instability. During the 1980s, cytogenetic and molecular examination of the fragile X site contributed significantly to disrupting the longstanding genetic dogma of intergeneration stability, and demonstrated the role of genomic dynamism in the development of disease-causing mutations.