DRD4 and post-traumatic stress disorder: Prior studies have linked polymorphisms in genes for DA transporter (DAT), DA receptor 4 (DRD4), DA receptor 3 (DRD3), and catechol-O-methyltransferase, with heightened risk for post-traumatic stress disorder (PTSD), greater PTSD symptom severity, impaired cognitive function, and impaired fear extinction in PTSD patients (Dragan and Oniszczenko, 2009; Drury et al., 2009; Kolassa et al., 2010; Norrholm et al., 2013; Wolf et al., 2014; Deslauriers et al., 2018; Havelka Mestrovic et al., 2018).