Before revision:  We simplify the study of steady states of these systems by explicit elimination of intermediate complexes  (inspired by Feliu-Wiuf'12 , 13 and Thomson-Gunawardena'09)and we  define an important subclass of MESSI systems with toric steady states .
After revision:  We simplify the study of steady states of these systems by explicit elimination of intermediate complexes (inspired by Feliu and Wiuf (2013a , 13 and Thomson-Gunawardena'09)and we define an important subclass of MESSI systems with toric steady states .