Researchers in Japan have discovered that a phage-derived enzyme called endolysin, which targets highly resistant biofilm-forming bacteria, could help restore the gut microbiota to mitigate acute graft-vs.-host disease. Acute graft-vs.-host disease (aGVHD) is a common complication for patients who undergo allogeneic hematopoietic cell transplantation (allo-HCT). Recent studies have highlighted the significant role of the microbiome in aGVHD, with dysbiosis contributing to its pathogenesis.