In psychiatry research, the accepted criteria which a biomarker must fulfill to be called an endophenotype include: 
# An endophenotype must segregate with illness in the population.
# An endophenotype must be heritable.
# An endophenotype must not be state-dependent (i.e., manifests whether illness is active or in remission).
# An endophenotype must co-segregate with illness within families.
# An endophenotype must be present at a higher rate within affected families than in the population. 
# An endophenotype must be amenable to reliable measurement, and be specific to the illness of interest.