Stress-inducible 72-kDa heat shock proteins (HSP70) were encoded on genes in multiple chromosomes. The expression of mRNA transcribed from the gene (HSP70-1) on chromosome 6 was studied using reverse transcript polymerase chain reaction in peripheral blood mononuclear cells of patients with different diseases. The deletion of 29 bp occurred in 5' noncoding and subsequent 133 bp in coding sequences of HSP70 mRNA in patients with major depression (n = 18), while normal subjects (n = 10) and patients with schizophrenia (n = 5), essential hypertension (n = 3), rheumatoid arthritis (n = 7), and Graves' disease (n = 3) had normal mRNA. No such deletion occurred in genomic DNA and no protein was translated from deleted mRNA. The allel-specific abnormal transcript of the HSP70 gene on chromosome 6 thus may underlie the altered stress and/or immune response in major depression.