We recently identified several individuals carrying a missense mutation (G329A; Arg 110-Gln) in the FUT7 gene encoding fucosyltransferase VII. This enzyme is involved in the biosynthesis of the sialyl Lewis x (Le x) epitope on human leukocytes, which has been identified as an important component of leukocyte ligands for E-and P-selectin. No enzyme activity was measurable in expression studies in COS-7 cells using the mutated FUT7 construct. One of the identified individuals carried this mutation homozygously. Flow cytometry analysis of polymorphonuclear leukocytes (PMN) from this individual showed a nearly complete absence of staining with mAbs directed against sialyl Le x and a diminished staining with an E-selectin IgG chimera. However, staining with P-selectin IgG chimera and Abs directed against P-selectin glycoprotein ligand-1 was not affected by the mutation. PMN from the homozygously mutated individual was further analyzed in an in vitro flow chamber assay. The number of rolling PMN and the rolling velocities on both E-and P-selectin were in the range of PMN from nonmutated individuals. FUT4 and FUT7 mRNA was quantified in PMN isolated from individuals carrying the FUT7 mutation. It was found that PMN from both FUT7 homozygously and heterozygously mutated individuals exhibited an elevated expression of FUT4 mRNA compared with PMN from FUT7 nonmutated individuals. The elevated expression of fucosyltransferase IV was reflected as an increased expression of the Le x and CD65s Ags on PMN from these individuals. The significance of the mutation was supported by transfection of BJAB cells.