The unique function of the gene egghead as a GDP-mannose: Glcβ1-Cer β1,4 mannosyltransferase is disclosed. The invention discloses isolated DNA molecules and DNA constructs encoding fragments of egghead and derivatives thereof by way of amino acid deletions, substitutions or insertions exhibiting egghead activity, as well as cloning and expression vectors including such DNA, cells transfected with vectors, and recombinant methods for providing egghead protein. Further, the invention discloses methods of obtaining β1,4-mannosylated glycosphingolipids by use of an enzymatically active egghead protein or by using cells stably transfected with a vector including DNA encoding an enzymatically active egghead protein as an expression system for recombinant production of such glycosphingolipids. Also a method for changing, altering or blocking the glycosphingolipid synthesis of cells by stably or transiently transfection with a vector including DNA encoding enzymatically active egghead protein. Furthermore, a novel method for stimulation of the immune system by cell surface presentation of βMan residues is disclosed.