KLK8 and bipolar disorder: KLK8 has also been implicated in schizophrenia, bipolar disorder (Izumi et al., 2008) and depression, in which patients with recurrent depression manifest higher KLK8 expression than first-episode patients (Talarowska et al., 2016; Bobińska et al., 2017; Starnawska et al., 2019).