RFLP analysis of the PvuII polymorphism. Lane 1, 4, 5: Two fragments of 850 bp and 450 bp for pp genotype, Lane 2: undigested PCR product of 1300 bp for PP genotype, Lane 3, 6: Three fragments of 1300 bp, 850 bp and 450 bp for Pp genotype, Lane 7: PCR product of external PCR. b: ϕX174 molecular size marker