Types of adult stem cells include hematopoietic stem cells and mesenchymal stem cells. Hematopoetic stem cells are found in the bone marrow and generate all cells of the immune system all blood cell types. Mesenchymal stem cells are found in umbilical cord blood, amniotic fluid, and adipose tissue and can generate a number of cell types, including osteoblasts, chondrocytes, and adipocytes. In medicine, adult stem cells are mostly commonly used in bone marrow transplants to treat many bone and blood cancers as well as some autoimmune diseases. (See Hematopoietic stem cell transplantation)  Of the types of adult stem cells have successfully been isolated and identified, only mesenchymal stem cells can successfully be grown in culture for long periods of time. Other adult stem cell types, such as hematopoietic stem cells, are difficult to grow and propagate in vitro. Identifying methods for maintaining hematopoietic stem cells in vitro is an active area of research. Thus, while mesenchymal stem cell lines exist, other types of adult stem cells that are grown in vitro can better be classified as primary cells. Induced pluripotent stem cell (iPSC) lines are pluripotent stem cells that have been generated from adult/somatic cells. The method of generating iPSCs was developed by Shinya Yamanaka's lab in 2006; his group demonstrated that the introduction of four specific genes could induce somatic cells to revert to a pluripotent stem cell state. Compared to embryonic stem-cell lines, iPSC lines are also pluripotent in nature but can be derived without the use of human embryos—a process that has raised ethical concerns. Furthermore, patient-specific iPSC cell lines can be generated—that is, cell lines that are genetically matched to an individual. Patient-specific iPSC lines have been generated for the purposes of studying diseases and for developing patient-specific medical therapies. Stem-cell lines are grown and maintained at specific temperature and atmospheric conditions (37 degrees Celsius and 5% CO2) in incubators. Culture conditions such as the cell growth medium and surface on which cells are grown vary widely depending on the specific stem cell line. Different biochemical factors can be added to the medium to control the cell phenotype—for example to keep stem cells in a pluripotent state or to differentiate them to a specific cell type.