Dopamine Receptor D4 (DRD4), a Dopamine Receptor, has been suggested to affect personality, substance dependence, cognition, and emotion. DRD4 binds the antipsychotic medications clozapine and spiperone. DRD4 knock-out mice are anatomically normal but display altered locomotion and sensitivity to drugs such as clozapine, cocaine, and others. The DRD4 gene contains extensive polymorphisms consisting of variable numbers of tandem repeats (VNTR) in sequences corresponding to the third cytoplasmic loop of the receptor; at least 27 different haplotypes encoding 20 different protein variants have been documented. The frequency of the different variants varies among population groups. Studies have suggested a link between VNTR and human personality traits (such as novelty seeking) and diseases (such as schizophrenia, bipolar affective disorder, and Tourette syndrome); however, considerable debates exist regarding the validity and significance of such associations. DRD4 expression has been reported in various regions of the brain as well as in adrenal gland, artery, eye, heart, kidney, placenta, spinal cord, testis, and vas deferens. ESTs have been isolated from a kidney library.