. "Two isoforms of adenylosuccinate synthetase, ADSS and ADSSL1, catalyze the reaction of IMP, aspartate, and GTP to form adenylosuccinate, GDP, and orthophosphate (Powell et al. 1992; Sun et al. 2005). ADSS is a homotetramer (unpublished crystallographic data - PDB 2V40) and ADSSL1 is inferred to be a tetramer likewise."^^ . . . .