For example, although primarily studied in affective disorders, the HPA axis is frequently found to be abnormal in schizophrenia. Plasma HVA, which reflects only 30% or more brain dopamine metabolism, is considered to be a good marker of central dopamine activity. On the other hand, plasma GABA, 95% of which comes from the brain, is highly understudied in both depression and schizophrenia. The limited pursuit of single neurotransmitter systems has not addressed major areas of clinical interest such as the difference between unipolar and bipolar depression.