Serum BDNF levels are reduced in depression (Molendijk et al. 2013), euthymic BD (Monteleone et al. 2008), acute mania (Machado-Vieira et al. 2007; Tramontina et al. 2009), and bipolar depression (Fernandes et al. 2009) and are lower with longer duration of illness (Kauer-Sant'Anna et al. 2009). Here, BDNF is linked to bipolar disorder.