CRISPR-Cas9 has been one of the most pivotal developments in 21st Century Genetics. To put it simply, CRISPR technology uses a "Guide RNA" to bind to a specific site of DNA. That RNA-DNA pair is then cleaved with an Endonuclease and a cut in the DNA chain is made. Selected/Desired genes can then be inserted into the cleaved portion of DNA allowing for novel protein expression. For agriculture this has been particularly well studied as it allows botanists to insert genes that make crops resistant to pathogens, tolerate droughts better, or grow uniquely colored/flavored products. The same pattern of applications have been used in animal agriculture, making many disease resistant breeds of fish, poultry, etc.
A Red Blood Cell with Sickle Cell Anemia
In medicine, gene therapy is becoming one of the hot topics. Recently, CRISPR-Cas9 was approved by the FDA for human trials to treat Sickle Cell Anemia. Sickle Cell Anemia is caused by a genetic mutation that inhibits the formation of hemoglobin. By manipulating the genomes of those with sickle cell anemia, scientists could theoretically let those afflicted produce normal hemoglobin, and in turn be cured of their condition.
A diagram on how CRISPR-Cas9 works
One worry I have is whether or not mutations associated with sickle cell anemia are pleiotropic. If they are there is a potential for patients to produce healthy hemoglobin but could potentially express other currently unknown problematic proteins. However, given the thoroughness of modern medical trials, this worry seems highly unlikely.