Polymerase chain reaction product of paired lymphocyte (L) and tumour (T) DNA from coamplification of glutathione S-transferase (GST)M1 (271 base pair) and interferon-β (170 base pair) genes. Lane 1 shows a homozygously present GSTM1 allele. Lane 2 shows a homozygously null-GSTM1 allele. M1 is ΦX174-HaeIII digested DNA marker. M2 is ΦX174-HinfI digested DNA marker.