Sensitivity and specificity of nested PCR-SSP typing. DNA from one individual (HLA-DRB1*0102, *1101/4; DRB3*0202/3) was mixed with DNA of known type (HLA-DRB1*1501, *0701; DRB4*0101; DRB5*01) at relative concentrations of 1%, 0.1%, 0.01%, and 0.001%. DNA mixtures were typed using nested PCR-SSP and products analyzed by agarose gel electrophoresis. All HLA alleles from the donor can be detected to a level of 0.001% (donor and recipient lanes are in bold type, with donor lanes also shaded. The lane yielding the false amplification is outlined).