GMPS and metastatic melanoma: In humans, guanosine monophosphate synthase (GMP synthetase and/or GMPS) is required for the de novo biosynthesis of GMP and has, among others, been reported to play a major role in the invasion and tumorigenicity of cells from some human metastatic melanomas, with increased levels of GMPS in these latter compared to what is observed for primary melanomas [2].