MPO genotypes determined by direct sequencing of PCR products. A DNA fragment including the -463 G/A base difference was amplified by PCR and directly sequenced using an internal primer. Positions of hexamers 1 and 2 (H1 and H2) are indicated in the sequence at left. The sequence of the opposite strand was determined using dideoxynucleotide C and T termination reactions as indicated at bottom. The SpSp homozygous genotype is identified by the G residue at -463 (arrow); the heterozygous SpN genotype has both G and A residues, and the homozygous NN genotype has only A.