Estimation of PCR and Southern blot sensitivity for detection of L428 cells admixed with normal PBMCs. (A and B) DNAs isolated from serial dilutions of L428 cells in normal PBMCs were amplified using V_H5FRI and L428rev1 (296 bp)/rev2 primers (287 bp). PCR products were separated on a 2% agarose gel and stained with ethidium bromide (A) and subjected to Southern blot hybridization using an internal FITC-labeled oligonucleotide (L428intern) and visualized by ECL (B). M, 100-bp DNA ladder. Negative controls: DNA from a healthy volunteer (PBMC*), water control after first and second (#) PCR.