CTLA4 and graft versus host disease: The FDA approved abatacept in late 2021 for GVHD prophylaxis in patients undergoing HCT from a matched unrelated donor (MUD) or 1 allele-mismatched unrelated donor (MMUD); thus, abatacept use in this case for a haploidentical HCT was off-label and offers an example of how the role of CTLA4-Ig blockade as GVHD prophylaxis, with further investigation, could potentially be expanded8.