Within the framework of developing a carbohydrate-based vaccine against cancer, we designed and prepared the MAG-Tn3, a fully synthetic immunogen based on the tumor-associated Tn antigen. The MAG-Tn3 is a glycopeptide associating Tn clusters with a pan-DR CD4+ T cell epitope, on a tetravalent backbone. It is a promising therapeutic vaccine against adenocarcinomas (breast, lung, and prostate cancer, among others). Our study demonstrates the feasibility of the synthesis of this complex glycopeptide as a drug-grade compound. Based on these results, and on successful in vivo experiments in mice and non-human primates, a phase I clinical trial for this vaccine candidate is scheduled to start in 2015 in patients with cancer.