Stem-cell therapy is the use of stem cells to treat or prevent a disease or condition. Bone marrow transplant is the most widely used stem-cell therapy, but some therapies derived from umbilical cord blood are also in use. Research is underway to develop various sources for stem cells, as well as to apply stem-cell treatments for neurodegenerative diseases and conditions such as diabetes and heart disease, among others. Stem-cell therapy has become controversial following developments such as the ability of scientists to isolate and culture embryonic stem cells, to create stem cells using somatic cell nuclear transfer and their use of techniques to create induced pluripotent stem cells. This controversy is often related to abortion politics and to human cloning. Additionally, efforts to market treatments based on transplant of stored umbilical cord blood have been controversial. Stem cells are being studied for a number of reasons. The molecules and exosomes released from stem cells are also being studied in an effort to make medications. In addition to the functions of the cells themselves, paracrine soluble factors produced by stem cells, known as the stem cell secretome, has been found to be another mechanism by which stem cell-based therapies mediate their effects in degenerative, auto-immune and inflammatory diseases.