In the course of this chapter we introduce one genetic condition – Rett syndrome. Our purpose is to exemplify the main analytic theme of this monograph: the multiple sites and versions in which this medical condition is enacted and represented. We also outline why this rare condition provides us with an analytic window onto wider issues in the sociology and anthropology of medical conditions, notably those with a genetic basis. We begin with a deceptively simple account of Rett syndrome derived from standard medical sources.