PENK and major depressive disorder: Thus, a coherent link emergesbetween specific genes, the variability of which is associatedboth with depression at the behavioral level and with a neurophysiologicalmarker of elevated depressive symptomatology.Information from Ivanov et al. (2019) further clarifies thebiological roles of these genes: (a) ADRA2B encodes the alpha-2B adrenergic receptor, a member of the G protein-coupledreceptor family; (b) TF encodes transferrin; (c) HCRTR2encodes hypocretin (orexin) receptor type 2; (d) WFS1 encodeswolframin; and (e) PENK encodes the proenkephalinprecursor protein.