 The reduction is thought to occur by mitochondrial proteins or microsomal single electron transfer proteins, though it may also occur by trans-membrane electron transport systems which reside outside the cell – implying that entry to the cancerous cells may not be required for the drug to be effective.In theory it is also possible for the ruthenium compounds to be oxidized back to its inactive form if it leaves the cancerous environment.This phenomenon remains a theory, and while demonstrated "in vitro", it has so far been difficult to prove experimentally "in vivo".