P CR Amplification of MMTV-like env gene sequences in the INER51 cell line. a) The first PCR reaction used the 1-3 primer sets to amplify a 665-bp product. The DNA was re-amplified using b) the 2N-3L primers to amplify a 191-bp product and c) the 2N-3N primers to amplify a 254-bp product. d) We used the 5L-3N primers to amplify a 656-bp product. The DNA was re-amplified e) using 2N-3L primers to yield a 191-bp product.