The activation of human CatSper channels by egg-derived steroids is essential for successful fertilisation. Disruptions in CatSper function are linked to male infertility. Understanding the molecular mechanisms underlying CatSper activation is crucial for developing improved treatments for infertility. Cryo-electron microscopy, a powerful tool for visualising macromolecular structures, can reveal the conformational changes in CatSper during activation. Photoaffinity labelling with a modified progesterone molecule will help to identify specific binding sites. This project aims to elucidate the precise mechanism of CatSper activation by progesterone by combining in situ cryo-electron tomography (Cryo-ET) with photoaffinity labelling. We will analyse the structure of the CatSper complex in the presence and absence of the photoaffinity-labeled progesterone, using our established protocols for human sperm sample preparation and cryo-ET data acquisition. Ultimately, this research will provide detailed structural insights into CatSper activation and contribute to the development of novel infertility treatments.