**8. Conclusions**

The speed and ease of genome engineering technologies has helped to overcome many of the limitations for the use of pig organs for xenotransplantation. Despite recent achievements, a key question remains: which combination of genetic modifications is most critical to make a pig organ useful for xenotransplantation? Ongoing experiments seek to address this question, but the answers are likely to be complex and dependent upon the type of organ, the specific immune mechanisms involved, and perhaps other factors that are not yet defined. It is very likely that the first set of genetic modifications of pigs used for xenotransplantation in humans will not be the final set, as the understanding of the mechanisms of xenorejection increases and better strategies developed to influence the human immune response. Continuing progress in genome engineering technologies of pigs will allow the creation of the more complex modifications necessary to meet these demands. Although much remains to be done, it is clear that given the current rate of progress, overcoming the crisis of human organ shortage with unlimited rejection-free porcine organs is rapidly growing closer to reality.
