DRD1 and schizophrenia: Furthermore, despite the difficulties of replicating genome wide association studies and candidate gene studies in schizophrenia, a number of genes in the dopamine pathway, including dopamine D1 receptor (DRD1), dopamine D2 receptor (DRD2), dopamine D5 receptor (DRD5), differential display clone 8 (DDC), catechol-O-methyltransferase (COMT) and solute carrier family 18 member 2 (SLC18A2), which is also a vesicular monoamine transporter, have been identified as associated with schizophrenia [6].