For example, a polymorphism of the M2-AChR encoding gene, CHRM2, is significantly correlated with development of MDD (12, 13) and MDD patients show reduced binding activity of M2-AChR, but not M1-, M3- and M4-AChR, in the dorsolateral prefrontal cortex (14, 15). Here, CHRM2 is linked to major depressive disorder.