To date, six human α3-fucosyltransferases (α1,3-FUTs) have been cloned. α3- fucosyltransferase-IX (Fuc-TIX, FUT9) is the newest member of the six human α3- FUTs, and has been cloned by an expression cloning method from a mouse brain cDNA library (Kudo et al. 1998). All the α1,3-FUT members, including those of Helicobacter pylori (Ge et al. 1997), possess the conserved amino acid stretch FxL/VxFENS/TxxxxYxTEK as a motif of α1,3-FUT. The phylogenetic tree of α1,3- FUTs indicates that the α1,3-FUT genes evolved by independent gene duplication between vertebrates and Caenorhabditis elegans. There are four clusters in the vertebrate α1,3-FUT gene family, corresponding to the FUT3-FUT5-FUT6 gene cluster, FUT4, FUT7, and FUT9 gene subfamilies. The FUT9 gene family seems to be the first to diverge from the ancestral gene (Kaneko et al. 1999a). The FUT9 gene does not cross-hybridize with the other five α1,3-FUT genes. The FUT9 amino acid sequence is quite different from those of the other five α1,3-FUTs which share highly homologous sequences. More interestingly, the amino acid sequence of FUT9 (Fut9) is very highly conserved between mouse and human. The degree of conservation is almost equivalent to that of the α-actin gene. This indicates that FUT9 is under a strong selective pressure for preservation during evolution. Mouse has only three functional α1,3- Fut genes, Fut4, Fut7, and Fut9, corresponding to human FUT4, FUT7, and FUT9 genes. FUT9 (Fut9) transfers Fuc from GDP-Fuc to the GlcNAc residue of type-2 chain with an α1,3-linkage, resulting in the synthesis of Lewis X (Lex) epitope, Galβ1-4(Fucα1- 3)GlcNAc-R. The FUT9 gene is localized at 6q16 in human (Kaneko et al. 1999b), and the mouse Fut9 gene was mapped at chromosome 4 (unpublished data, 1998).