It is notable that D2R-KO mice were profoundly impaired when reward uncertainty was added to the task (i.e., TAB task) even with prolonged training, which is in line with impaired performance of Parkinson's disease (PD) patients in probabilistic learning tasks (Knowlton et al., 1996; Shohamy et al., 2004). The gene discussed is DRD2; the disease is Parkinson disease.