Hyper-acute rejection is an immunological reaction elicited against the disaccharide galactose-α-1,3-galactose following xeno-transplantation of organs into human recipients. In a recent Cell paper, Zhong et al. showed that this reaction can also be used to cause regression of established tumors in latestage cancer patients by expressing the α1,3 galactosyltransferase gene from a novel oncolytic virus.