The family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (GalNAc-transferases, GALNTs) carry out the addition of N-acetylgalactosamine (GalNAc) on serine or threonine residues of proteins, especially mucins. This is the initial reaction in the formation of O-linked oligosaccharide biosynthesis (Guo et al. 2002). Defects in one of the GALNT family, GALNT12, can result in decreased glycosylation of mucins, mainly expressed in the digestive organs such as the stomach, small intestine and colon, and may play a role in colorectal cancer 1 (CRCS1; MIM:608812). CRCS1 is a complex disease characterised by malignant lesions arising from the inner walls of the colon and rectum. Mutations implicated in CRCS1 are M1I, T491M, Y395* (Guda et al. 2009), D303N and Y396C (Clarke et al. 2012).