C1GALT1C1 and carcinoma: To this date, two main mechanisms leading to STn biosynthesis have been described: i) the overexpression of ST6GALNAC1 [22, 50], which is commonly observed in human carcinomas [15, 50]; ii) loss-of-function of C1GALT1 chaperone Cosmc [51, 52], which is considered a rare event [53].