The presence of the FORS1 antigen on red blood cells was discovered relatively recently, and in 2012, the International Society of Blood Transfusion (ISBT) acknowledged FORS as blood group system number 031. This rare antigen is carried by a glycosphingolipid and formed by elongation of the P antigen. Most people have naturally occurring anti-FORS1 in their plasma. The clinical significance of these antibodies is unknown in the transfusion setting, but they can hemolyze FORS1+ erythrocytes in the presence of complement in vitro. First believed to be part of the ABO system, it was later shown that the gene encoding the glycosyltransferase giving rise to FORS1 expression is