Fig. 1.

Simplified schematic representation of the endophenotype concept in psychiatric genetics. Many genes (depicted in an oversimplified way as gene A to I) are involved in causing a disease symptom and the categorical disease phenotype. A reduced number of genes is involved in disease related endophenotypes, like ‘Function of a brain unit’ (Level 1), as assessed through performance on neuropsychological tests; even less genes play a role if an endophenotype at levels 2 or 3 is used for genetic association studies. On level 2, endophenotypes measured through (structural and functional) neuroimaging are shown, on level 3 cellular behavior, assessed through e.g. assays measuring migration, apoptosis or cell division rates are shown