PDE4B and schizophrenia: Indeed, there have been accumulating studies investigating association of PDE4B variations with predisposition to schizophrenia across multi-ethnic populations, including Caucasian populations (Europeans [19–21], Caucasian Canadians [22] and Caucasian Americans [23]), Asian populations (Indians [18], Japanese [24], Chinese [25] and Koreans [26]), and African populations (African Americans [23]).