Is sex still binary?

In this perspective article we discuss the limitations of sex as a binary concept and how it is challenged by medical developments and a better understanding of gender diversity. Recent data indicate that sex is not a simple binary classification based solely on genitalia at birth or reproductive capacity but encompasses various biological characteristics such as chromosomes, hormones, and secondary sexual characteristics. The existence of individuals with differences in sex development (DSD) who do not fit typical male or female categories further demonstrates the complexity of sex. We argue that the belief that sex is strictly binary based on gametes is insufficient, as there are multiple levels of sex beyond reproductivity. We also explore the role of sex in sex determination, gene expression, brain development, and behavioural patterns and emphasize the importance of recognizing sex diversity in personalized medicine, as sex can influence disease presentation, drug response, and treatment effectiveness. Finally, we call for an inter- and transdisciplinary approach to study sex diversity and develop new categories and methodologies that go beyond a binary model.