12 pg of HPV-18 plasmid DNA, completely CpG methylated with SssI methyltransferase, were PCR amplified with methylation specific primers (band in lane 2) and primers specific for unmethylated DNA (no band in lane 6). There is no signal with methylated DNA targeted by the primer pair specific for unmethylated DNA (lane 3), while the unmethylated DNA became amplified with these primers (lane 7). Lane 1: size marker, lanes 4,5,8,9: negative control (water).