Often a patient’s DNA is sequenced in order to find a mutation in a gene that could potentially cause his or her disease symptoms. To determine if loss of function of that gene could cause the symptoms seen in the patient, the same gene is mutated or “knocked-out” in zebrafish, and then the fish are examined for similar symptoms. Although it is much more difficult to do, the exact mutation that the patient has can be introduced into zebrafish as well—this is called a “knock-in”.