Ernst Holler, MD, University Hospital Regensburg, Regensburg, Germany, gives an overview of recent research into biomarkers and prognostication for graft-versus-host disease (GvHD), highlighting work by the MAJIC consortium. Prof. Holler reports that risk-adapted clinical trials are underway, which aim to identify GvHD early through the identification of biomarkers, enabling the optimal treatment of patients at high risk of GvHD, and preventing the over-treatment of patients who are at low risk. Prof. Holler also discusses how early loss of commensal bacteria in the gut is a biomarker for severe GvHD, commenting on how this can be monitored in clinical practice and used to guide treatment decisions. This interview took place at the 6th Congress on Controversies in Stem Cell Transplantation and Cellular Therapies (COSTEM), which took place virtually.