Platelet monoamine oxidase (MAO) activity, determined in 102 patients
with chronic schizophrenia, 223 first-degree relatives, and 88 normal
control subjects, was shown to be a heritable and stable trait and was
significantly lower in patients than in normal control subjects. Within
families, MAO activity distinguished ill from well relatives. However, the
considerable overlap in enzyme activity between affected and unaffected
individuals limits the usefulness of low MAO activity as a major risk
factor in schizophrenia.