PENK and substance dependence: Short tandem repeats and VNTRs in the cannabinoid receptor 1 (CNR1), glutamate ionotropic receptor NMDA type subunit 2A (GRIN2A), prodynorphin (PDYN), and proenkephalin (PENK) genes were reported to be associated with the vulnerability to schizophrenia and substance dependence (Comings et al., 1997; Comings et al., 1999; Chen et al., 2002; Itokawa et al., 2003).