Complex sentence: All of the proteins involved in neurotransmission are a small fraction of the more than 100,000 proteins in the brain, each of which may be a target for specific therapy.
Split into simpler sentences: All of the proteins involved in neurotransmission are a small fraction of the more than 100,000 proteins in the brain. Thus there are many proteins not directly in the path of signal transduction, any of which may still be a target for specific therapy.