 Your immune system is made up of an army of cells that protect you from infections Like the virus that causes the common cold or the bacteria that cause strep throat Different types of immune cells have different special functions like recognizing an infection killing infected cells or closing up open wounds To be able to develop and function cells of the immune system follow a set of instructions and turn on specific genes This is much like how you would use a blueprint when building a house if a detail is left out of the blueprint You may end up with a house without a door Babies who are born with mutations in the important immune system genes lack the correct blueprint and as a result They can't develop a normal immune system Because their immune cells cannot fight off foreign invaders even minor infections can be deadly These babies have what is called severe combined immunodeficiency or skid Skid is commonly known as bubble boy disease because at one time the only way to manage the disease was to isolate the sick child in a sterile environment left untreated skid babies typically die within their first two years of life Fortunately researchers have worked hard trying to fix the immune systems of skid babies and continue to develop better stem cell transplant strategies to cure skid Before we explain the therapies, let's discuss how the immune system develops a special type of cell lives in our bone marrow called a hematopoietic stem cell or HSC these cells can produce all of the different types of immune cells However, if the HSC has a mutation in a critical immune system gene all of the immune cells It produces will be defective Scientists are trying several complementary approaches one fixing the gene mutation in the HSC and Two getting rid of all of the mutated HSCs and replacing them with normal ones Let's start by discussing how we can go about fixing the mutation Researchers in the group of Don Cohn at UCLA and others have been tackling this problem since the 1990s They have developed a strategy to repair a gene mutation in HSCs from skid babies a method we'll call gene addition First bone marrow from skid patients is removed and the stem cells are purified Then researchers make a piece of DNA that has the correct Non-mutated DNA sequence in the gene that needs fixing They then insert this normal DNA into the HSCs from skid patients With the normal version of the gene the stem cells and the specialized immune cells They produce will all have a normal copy of the gene and will make normal proteins Patients are given a low dose of chemotherapy to make space for the new cells and then the repaired HSCs are transplanted Incredibly this method works the first skid trial using gene addition was conducted over 20 years ago and Researchers have improved on the technique a great deal The last 30 skid babies treated with gene addition have developed a good working immune system that has stayed stable over time Another team of researchers at Stanford have developed a complementary approach, which we'll call HSC replacement In order to allow the repaired HSCs to work the defective HSCs must first be removed as we mentioned currently Skid children receive chemotherapy to remove the defective HSC However, chemotherapy is toxic. So these researchers want to remove the defective HSC in a safer way Their method involves giving patients an antibody, which is a special protein that recognizes other proteins in a very specific manner This particular antibody targets and eliminates HSCs from the body Researchers can then transplant stem cells from a healthy person or HSCs repaired using gene addition back into the patient By eliminating the need for chemotherapy before the healthy cells are transplanted There will be fewer adverse side effects from this treatment by using these two complementary approaches correcting the mutated gene in a patient's own stem cells and Replacing their mutated HSCs with healthy ones Scientists and physicians are helping skid babies build strong functioning immune systems and live long healthy lives and What's even more exciting is that similar gene therapy strategies might work for other diseases Known to be caused by mutations in a single gene such as sickle cell anemia, cystic fibrosis and muscular dystrophy So far both the gene therapy and HSC replacement approaches have shown promising pre-clinical and clinical data However, it is important to keep in mind that these results are still early and the therapies must be tested further As we build on the progress enabled by California's Proposition 71 and the funding agency Created the California Institute of Regenerative Medicine. We must keep the momentum going We now have more advanced tools to develop better ways to treat devastating genetic diseases such as skid Only with continued support for biomedical research Can we increase our understanding of the therapeutic potential of stem cells and Translate that understanding into meaningful treatments that help give patients their lives back