The de novo synthesis pathway for GDP-L-fucose is a two step pathway starting from GDP-mannose. In the first step, GDP-mannose (GDP-Man) is dehydrated to GDP-4-dehydro-6-deoxy-alpha-D-mannose (GDP-DHDMan) by GDP-mannose 4,6 dehydratase (GMDS) (Sullivan et al. 1998, Ohyama et al. 1998). Fucosylation is one of the most important oligosaccharide modifications in cancer and inflammation. Defects in GMDS are involved in the progression of colorectal cancer (Nakayama et al. 2013).