The purpose of this study is to collect two blood specimens and information about participants who are receiving a standard of care bone marrow or stem cell transplant to use and help better understand graft-versus-host disease (GVHD). One potential new way to help researchers better understand GVHD is through a biomarker (a substance found in your blood/tissue) called ST2. The new information learned from this study could then be used in future research for patients undergoing stem cell transplants. This is a non-interventional study (no drugs involved).