In this lecture, Sapolsky talks briefly about the genetic component but at a super high level "good/bad" version of a gene.

Depression is a genetic disorder. It has some degree of heritability. 50% identical twins, 25% full sibling. So again we have the whole nature-nurture interaction. Have the bad gene (a serotonin one), add in major stressors and uh-oh. A 30 fold increase in the likelihood of depression at the extreme. Oh, and glucocorticoids regulate the expression of the gene.

Anyone know what gene he is referring to?