Mesenchymal stem cells (MSCs) are defined as self-renewing, multipotent progenitor cells with potential to differentiate into other cell types of mesodermal origin, such as adipocytes, osteocytes, and chondrocytes. Mesenchymal and Tissue Stem Cell Committee of the International Society for Cellular Therapy developed the minimal criteria for definition of MSCs which is as follows: first, adherence to plastic; Second, positivity for the cell-surface molecules CD105, CD73, and CD90 and negativity for CD45, CD34, CD14 or CD11b, CD79a or CD19, and human leukocyte antigen (HLA)-DR; and third, the ability to differentiate into osteoblasts, adipocytes, and chondroblasts under standard in vitro differentiation conditions. Since Le Blanc and colleagues first reported successful treatment of a patient with severe acute GVHD using third-party haploidentical MSCs in 2004, many clinical trials worldwide have described the benefits of MSC therapy in GVHD.