Considering the emerging link between impaired cerebellar function and mania, and that decreased serum BDNF is a biomarker of bipolar disorder, the decrease in cerebellar BDNF expression in the Camkk2 and Camk4 null mice suggests that defects in CaMKK2-CaMK4 signalling within the cerebellum may be involved in triggering manic behaviour [46–49]. Here, BDNF is linked to bipolar disorder.