Regulatory T cells (Tregs) have been found to be important mediators of tolerance after experimental hematopoietic SCT, and have been used to prevent and treat GVHD in murine models [4] . While there is great interest in using Tregs therapeutically in clinical transplantation, just the presence and number of Tregs could serve as a useful biomarker for GVHD. The study by Magenau et al. uses flow cytometry to measure CD41/CD25hi/FOXP31 Tregs in 215 patients with (n 5 60) and without (n 5 155) GVHD.