Germline therapy is the intentional transfer of genes into germline cells (egg cells or sperm). It aims to prevent diseases resulting from a genetic defect in the offspring of the carrier of the genetic defect. Germline therapy thus primarily aims at the prevention of serious, gene-based hereditary diseases. 

Targeted germline intervention is prohibited by law in Germany and many other countries. The ban is firstly based on the unforeseeable consequences for possible offspring, since, for example, undesirable changes in the genome cannot be reliably ruled out and could be passed on over generations. Secondly, research into such a procedure would result in a high consumption of embryos and human experiments with possibly unconscionable processes could not be completely ruled out. Even if the goal of germline intervention is fundamentally justifiable, the research required for it would not be ethically warranted at present. In addition to the incalculable safety risks, there is also concern that the development of such methods could prepare the ground for targeted, improper modifications of the human genome. Even a legally very limited authorization of germline therapy for certain serious diseases would probably not adequately control the risk of misuse, as it is questionable according to which criteria a line should be drawn between serious and less serious diseases and disabilities. A broad, controversial debate on this topic has never really died down within the scientific community. There are approaches that reinforce the prohibition of germline intervention, as well as those that consider germline therapy not only permitted but even necessary under certain conditions, i.e. if a safe and reliable technique were available. 

Chinese scientist He Jiankui caused outrage in the scientific community and the general public when he announced in November 2018 the birth of two babies that he had genetically modified. He Jiankui claimed to have used CRISPR/Cas9 to modify the embryos' CCR5 gene to make them immune to the HI virus transmitted by the father. Jiankui allegedly implanted the two edited embryos into a woman who then gave birth to two living girls. He Jiankui's actions were roundly condemned for several reasons. On the one hand, it cannot be ruled out that the modification of the CCR5 gene could have other, more serious effects, since not all functions of this gene are fully known at present. On the other hand, the necessity of this intervention is highly controversial, as safe methods already exist to prevent the transmission of the HI virus from father to child. With his experiment, Jiankui also knowingly circumvented a number of ethical and scientific guidelines. He was suspended from the Southern University of Science in Shenzhen, where he worked. As the genetic editing of human embryos for the purpose of reproduction is also prohibited by law in China, Jiankui was prosecuted and convicted.  

The case prompted many researchers to call for a global moratorium on germline interventions, according to which all nations should voluntarily commit themselves to refrain from the clinical use of germline modifications to sperm, oocytes and embryos. As early as 2017, the German Ethics Council issued an ad-hoc recommendation calling for an international political discourse and international regulations on germline interventions in humans. This was followed in May 2019 by a detailed statement by the German Ethics Council, which also advocated an international moratorium. It stated that although ethical analysis did not show any categorical inviolability of the human germline, germline interventions were currently considered ethically irresponsible because of their incalculable risks.