Infertility affects 10-15% of couples worldwide. Depending on the cause of infertility, treatment ranges from medications to stimulate ovulation through to more complex assisted reproductive technologies (ARTs), such as in vitro fertilisation (IVF). These fertility treatments have been reasonably successful; however, they all rely on decades-old medications that promote or suppress hormone activity (e.g., Clomid, Decapeptyl and Gonal-F). While the hormones of the hypothalamic-pituitary-gonadal axis are certainly critical in controlling female fertility, they are not the only factors involved. Ovarian follicle growth and development also depends upon the activity of transforming growth factor-ß (TGF-ß) proteins, including Anti-Müllerian Hormone (AMH), growth differentiation factor 9 (GDF9), bone morphogenetic protein 15 (BMP15), inhibin A and inhibin B. We have modified each of these proteins with the aim to provide new opportunities to regulate female fertility.