We report a null mutation in the first exon of the human dopamine D4 receptor (DRD4) gene. The mutation is predicted to result in a truncated non-functional protein and is the first natural nonsense mutation found in a human dopamine receptor gene. It occurs with a frequency of about 2% in the general population. The distribution of the mutation was found to be similar in healthy controls and patients suffering from psychiatric diseases which included schizophrenia, bipolar affective disorder and Tourette's syndrome, indicating that heterozygosity for this mutation in the DRD4 gene is not causally related to major psychiatric diseases. We also identified an adult male who is homozygous for this mutation. He shows no symptoms of major psychiatric illness, but he displays somatic ailments including acousticous neurinoma, obesity and some disturbances of the autonomic nervous system. Some of these symptoms might be related to the absence of functional DRD4 protein.