The FORS blood group system consists of one antigen (rare) and the carbohydrate structure is present in the RBC membrane in the globo-series biosynthetic pathway. The terminal carbohydrate moiety is an extension of the P antigen (globoside, Gb4). The glycosyltransferase that synthesizes the FORS1 antigen consists of 347 amino acids and is considered non-functional in most individuals. In antigen positive individuals the glycosyltransferase is activated due to a point mutation in exon 7.