For the most part, however, studies that have failed to demonstrate that a reduction in Treg cell frequency and/or absolute numbers is associated with increased GVHD severity have relied on CD25 expression to delineate Treg cell populations, whereas those that have reported a positive correlation have tended to employ Foxp3 expression as a readout for this Treg cell population. Here, FOXP3 is linked to graft versus host disease.