Crow's viral hypothesis of schizophrenia proposes that psychosis may be the result of mutagenesis caused by viral integration or transposition in human genomic DNA. Molecular genetic techniques can be used to systematically investigate this hypothesis. In a study of genomic lymphocyte DNA unexpected DNA polymorphisms which were probably insertions and deletions were found in specific human genomic retroviral (proviral) related sequences. However these changes were found exclusively in normal Icelandic individuals and are probably of evolutionary origin. The extent to which human retroviral insertion and deletion has taken place and the mobility of such sequences will help in understanding their evolutionary origin and might provide a source of polymorphic marker sequences that could be used in genetic linkage studies of disease.