In recent decades, Iran has witnessed radical transformations concerning the conceptualization of and procedural standards for changing sex. Psychologists, medical and legal practitioners, law enforcement officials, and scholars of fiqh have debated the advisability (in debates among health and legal professionals) or the permissibility (among scholars of fiqh) of sex-change. This article asks what historical transformations of the concept of jins/genus have informed the debates and enabled the contemporary dominant concepts and practices that shape them. How has jins come to mean sex and how does this matter? The article first maps out the historical genealogy of these reconfigurations. What were some of the 19th-and pre-19th-century concepts that could be considered disparate precedents to this cluster around sex/jins? It then reviews some of the late-19th-and 20th-century reshaping of biomedical knowledge and marital practices that have contributed to the contemporary meanings of jins.