Imagine two people are listening to music. What are the odds that they are listening to the exact same playlist? Probably pretty low. After all, everyone has very different tastes in music. Now, what are the odds that your body will need the exact same medical care and treatment as another person's body? Even lower. As we go through our lives, each of us will have very different needs for our own healthcare. Scientists and doctors are constantly researching ways to make medicine more personalized. One way they are doing this is by researching stem cells. Stem cells are cells that are undifferentiated, meaning they do not have a specific job or function. While skin cells protect your body, muscle cells contract, and nerve cells send signals, stem cells do not have any specific structures or functions. Stem cells do have the potential to become all other kinds of cells in your body. Your body uses stem cells to replace worn-out cells when they die. For example, you completely replace the lining of your intestines every four days. Stem cells beneath the lining of your intestines replace these cells as they wear out. Scientists hope that stem cells could be used to create a very special kind of personalized medicine in which we could replace your own body parts with, well, your own body parts. Stem cell researchers are working hard to find ways in which to use stem cells to create new tissue to replace the parts of organs that are damaged by injury or disease. Using stem cells to replace damaged bodily tissue is called regenerative medicine. For example, scientists currently use stem cells to treat patients with blood diseases such as leukemia. Leukemia is a form of cancer that affects your bone marrow. Bone marrow is the spongy tissue inside your bones where your blood cells are created. In leukemia, some of the cells inside your bone marrow grow uncontrollably, crowding out the healthy stem cells that form your blood cells. Some leukemia patients can receive a stem cell transplant. These new stem cells will create the blood cells needed by the patient's body. There are actually multiple kinds of stem cells that scientists can use for medical treatments and research. Adult stem cells or tissue-specific stem cells are found in small numbers in most of your body's tissues. Tissue-specific stem cells replace the existing cells in your organs as they wear out and die. Embryonic stem cells are created from leftover embryos that are willingly donated by patients from fertility clinics. Unlike tissue-specific stem cells, embryonic stem cells are pluripotent. This means that they can be grown into any kind of tissue in the body. A third kind of stem cells is called induced pluripotent stem cells. These are regular skin, fat, liver, or other cells that scientists have changed to behave like embryonic stem cells. Like embryonic stem cells, they, too, can become any kind of cell in the body. While scientists and doctors hope to use all of these kinds of stem cells to create new tissue to heal your body, they can also use stem cells to help understand how the body works. Scientists can watch stem cells develop into tissue to understand the mechnanisms that the body uses to create new tissue in a controlled and regulated way. Scientists hope that with more research, they can not only develop specialized medicine that is specific to your body but also better understand how your body functions, both when it's healthy and when it's not.