One out of three couples remain childless after treatments with current assisted reproductive technologies. A challenging cause is the absence of functional gametes in these couples. Therefore, the need for artificial gametes is becoming increasingly important. This proposal will tackle this need by creating de novo oocytes through diploid cell haploidization. Hereby, a diploid nucleus is injected into an enucleated oocyte. Subsequent induction of meiosis yields reconstructed haploid oocytes. Haploidization will be investigated in both the mouse and human model. The protocol will be optimised on several levels in order to increase the efficiency and safety. Firstly, epigenetic reprogramming will be stimulated by the overexpression of KDM4D and KDM1B for the first time in this setting. Secondly, donor cell nuclei with a different epigenetic status will be compared (cumulus cells, fibroblasts, iPSCs and PGCLCs). We hypothesize that donor cells which are more reprogrammed to an embryonic state, will result in a higher efficiency. Lastly, the influence of the recipient ooplasm stage on haploidization will be investigated by using both immature and mature recipient oocytes. The efficiency and safety will be monitored through next generation sequencing based methods. Immunocytochemistry will provide valuable insights on the mechanics of haploidization. Overall, this project will lay the foundation of an alternative way for obtaining artificial oocytes in both mouse and human.