In February 2016, British scientists were given permission by regulators to genetically modify human embryos by using CRISPR-Cas9 and related techniques on condition that the embryos were destroyed in seven days. In June 2016, the US National Academies of Sciences, Engineering, and Medicine released a report on their "Recommendations for Responsible Conduct" of gene drives. Models suggest that extinction-oriented gene drives will wipe out target species and that drives could reach populations beyond the target given minimal connectivity between them. Kevin M. Esvelt stated that an open conversation was needed around the safety of gene drives: "In our view, it is wise to assume that invasive and self-propagating gene drive systems are likely to spread to every population of the target species throughout the world. Accordingly, they should only be built to combat true plagues such as malaria, for which we have few adequate countermeasures and that offer a realistic path towards an international agreement to deploy among all affected nations.". He moved to an open model for his own research on using gene drive to eradicate Lyme disease in Nantucket and Martha's Vineyard. Esvelt and colleagues suggested that CRISPR could be used to save endangered wildlife from extinction. Esvelt later retracted his support for the idea, except for extremely hazardous populations such as malaria-carrying mosquitoes and isolated islands that would prevent the drive from spreading beyond the target area. Austin Burt, an evolutionary geneticist at Imperial College London, introduced the possibility of conducting gene drives based on natural homing endonuclease selfish genetic elements in 2003. Researchers had already shown that such genes could act selfishly to spread rapidly over successive generations. Burt suggested that gene drives might be used to prevent a mosquito population from transmitting the malaria parasite or to crash a mosquito population. Gene drives based on homing endonucleases have been demonstrated in the laboratory in transgenic populations of mosquitoes and fruit flies. However, homing endonucleases are sequence-specific. Altering their specificity to target other sequences of interest remains a major challenge. The possible applications of gene drive remained limited until the discovery of CRISPR and associated RNA-guided endonucleases such as Cas9 and Cpf1. In June 2014, the World Health Organization (WHO) Special Programme for Research and Training in Tropical Diseases issued guidelines for evaluating genetically modified mosquitoes. In 2013 the European Food Safety Authority issued a protocol for environmental assessments of all genetically modified organisms. Target Malaria, a project funded by the Bill and Melinda Gates Foundation, invested $75 million in gene drive technology.