PCDH11X and major depressive disorder: Mushroom spine dendrites have greatly elevated length in cell lines that respond to bupropion treatment. Non-responsive lines have a non-significant reduction in dendrite length. In MDD cell lines derived from patients that did not respond to SSRIs, lowered expression of protocadherin-α genes resulted in altered neurite growth. Knockdown of protocadherin-α resulted in improved neurite length.