It has a larger sample size than existing studies; it includes patients from three diagnostic groups (major depression [MDD], bipolar disorder, and schizophrenia, as well as suicides and non-suicides within each group), and uses three complementary techniques: NeuN to assess all neurons, and TPH immunohistochemistry and 5-HT1AR mRNA in situ hybridization (ISH) as markers of serotonergic neurons. Here, TPH1 is linked to major depressive disorder.