Glycosylation reactions are of great biological importance to both prokaryotes and eukaryotes, and require the coordinate action of a large number of glycosyltransferases. These enzymes have high donor and acceptor substrate specificities, and are in general limited to catalysis of one unique glycosidic linkage. Emerging evidence indicates that formation of many glycosidic linkages is covered by large homologous transferase gene families, and that the existence of multiple ezyme isoforms provides a degree if redudancy, as well as higher level of regulation of the glycoforms synthesized. More precisely, the study of the initial step of the mucin-type O-glycosylation reaction is viewed here, i.e . the in vitro and in vivo specificities of the family of UDP-GalNAc: peptide N-acetylgalactosaminyltransferase isoforms (GaNTase¹- T1 to GaNTase-T9) are analysed on the basis of reactivity and/or inhibitory activity on mucin peptide motifs, the structrual  features, calatytic properties, genomic organization, and genetic expression patterns of the different members of GaNTase family are also detailed. Results from substrate binding studies suggest that (ii)multiple isoforms of GaNTases are present in various issues, and in particular glycopeptide: GaNTases (gpGaNTases); (i) the catalyzed trasfer proceed via an ordered sequential mechanism to rule in favour of particular attachment site density depending on mucin-type molecules.