How genes and DNA affect our health. The CRISPR system gives us the ability to edit DNA even in humans. Based on a natural system where cells protect themselves from infection from viruses, scientists discovered how the CRISPR system works using the Cas9 protein and how this became a way to engineer living cells, adjusted to target specific parts of DNA, precisely replacing a mutant gene with a healthy copy by adding desired DNA. This system of genome engineering can be used in developing drugs, improving agriculture and, in the future, in human stem cells that are used in every part of the body, as well as in embryo cells. The CRISPR system can be used simultaneously in many genes for many different purposes.