diff --git a/reactics-smt/rs/reaction_system_with_concentrations_param.py b/reactics-smt/rs/reaction_system_with_concentrations_param.py
index 29a318b..54a8695 100644
--- a/reactics-smt/rs/reaction_system_with_concentrations_param.py
+++ b/reactics-smt/rs/reaction_system_with_concentrations_param.py
@@ -98,6 +98,24 @@ class ReactionSystemWithConcentrationsParam(ReactionSystem):
 
         return False
 
+    def is_valid_concentration_for_entity(self, entity, level):
+        """Checks the concentration level for entity"""
+        e = self.get_entity_id(entity)
+        try:
+            max_conc = self.max_conc_per_ent[e]
+        except KeyError as _:
+            return True
+
+        if level > max_conc:
+            return False
+
+        return True
+
+    def terminate_on_invalid_concentration(self, entity, level):
+        if not self.is_valid_concentration_for_entity(entity, level):
+            print("FATAL. Invalid entity concentration: {:s}={:d}".format(entity, level))
+            exit(1)
+            
     def get_state_ids(self, state):
         """Returns entities of the given state without levels"""
         return [self.get_entity_id(e) for e in state]
@@ -124,6 +142,7 @@ class ReactionSystemWithConcentrationsParam(ReactionSystem):
                 self.is_valid_entity_with_concentration(r)
                 self.has_non_zero_concentration(r)
                 entity, level = r
+                self.terminate_on_invalid_concentration(entity, level)
                 reactants.append((self.get_entity_id(entity), level))
                 if self.max_concentration < level:
                     self.max_concentration = level
@@ -139,6 +158,7 @@ class ReactionSystemWithConcentrationsParam(ReactionSystem):
                 self.is_valid_entity_with_concentration(i)
                 self.has_non_zero_concentration(i)
                 entity, level = i
+                self.terminate_on_invalid_concentration(entity, level)
                 inhibitors.append((self.get_entity_id(entity), level))
                 if self.max_concentration < level:
                     self.max_concentration = level
@@ -154,6 +174,7 @@ class ReactionSystemWithConcentrationsParam(ReactionSystem):
                 self.is_valid_entity_with_concentration(p)
                 self.has_non_zero_concentration(p)
                 entity, level = p
+                self.terminate_on_invalid_concentration(entity, level)
                 products.append((self.get_entity_id(entity), level))
 
         return reactants, inhibitors, products
diff --git a/reactics-smt/rs_testing.py b/reactics-smt/rs_testing.py
index 81d1de3..4d1b57e 100644
--- a/reactics-smt/rs_testing.py
+++ b/reactics-smt/rs_testing.py
@@ -26,9 +26,9 @@ def run_tests(cmd_args):
     # heat_shock_response_param(cmd_args)
     # simple_param(cmd_args)
 
-    param_gene_expression(cmd_args)
+    # param_gene_expression(cmd_args)
 
-    # mutex_bench_main(cmd_args)
+    mutex_bench_main(cmd_args)
 
 def mutex_bench_main(cmd_args):
 
@@ -76,7 +76,8 @@ def mutex_param_bench(cmd_args):
             r.add_bg_set_entity(E(ent,i))
 
     for ent in shared_entities:
-        r.add_bg_set_entity((ent, 1))
+        max_conc = 2 if ent == "lock" else 1
+        r.add_bg_set_entity((ent, max_conc))
     
     ###################################################
     
@@ -94,7 +95,7 @@ def mutex_param_bench(cmd_args):
         r.add_reaction([E("req",i)],[E("act",i)],[E("req",i)])
         
         enter_inhib = [E("act",j) for j in range(n_proc) if i != j] + [("lock",1)]
-        r.add_reaction([E("req",i),E("act",i)],enter_inhib,[E("in",i), ("lock",1)])
+        r.add_reaction([E("req",i),E("act",i)],enter_inhib,[E("in",i), ("lock",2)])
         
         r.add_reaction([E("in",i),E("act",i)],Inhib,[E("out",i), ("done",1)])
         r.add_reaction([E("in",i)],[E("act",i)],[E("in",i)])