Abstract One of the frontiers in cancer personalized-medicine aims at glycosylation. Cells are covered with a dense sugar coat of glycolipids, glycoproteins and free glycans. In cancer, the characteristic cell surface glycosylation is frequently transformed due to altered expression of glycan-modifying enzymes. This often leads to aberrant expression of sialic acids (Sia) that cap glycan-chains. Additionally, dietary intake of the non-human Sia N -glycolylneuraminic acid (Neu5Gc) leads to natural metabolic-glycoengineering of human carcinomas that accumulate and express Neu5Gc. This Sia provokes a polyclonal anti-Neu5Gc xeno-autoantibodies response that can exacerbate cancer. This review highlights cancer-associated changes in Sia expression and their potential for personalized-theranostics.