The family of beta 4-galactosyltransferases (B4GALTs) is composed by at least six known members with different Km and acceptor specificities (Guo et al. 2001) that probably originated by gene duplication (Lo et al. 1998). They mediate the transfer of galactose to N-glycan structures, in this case, to elongate an antenna with a keratan chain. B4GALT1 is associated with Congenital Disorder of Glycosylation of type IId (MIM:607091) (Hansske et al. 2002), and is expressed as two splicing isoforms of which only one is localized in the Golgi system (Lopez et al. 1991, Schaub et al. 2006).