Clinical data and longitudinal serum samples from the Mount Sinai Acute GVHD International Consortium (MAGIC) were used to develop MAGIC Algorithm Probability (MAP) that accurately predicts the risk of severe GVHD and non-relapse mortality. Dr. Ferrara explains how to interpret serum MAPs before and after GVHD treatment, the relationship of the serum MAP to GI GVHD pathology, and how MAPs can guide treatment for acute GVHD.