sentence: We first determined the effect of HPV16 E7 expression on MAP Kinase activity by transducing immortalized NIH3T3 cells or human foreskin kerotinocytes ( HFK ) , the natural host of the virus , with a recombinant retrovirus expressing non tagged , wild-type HPV16 E7 and measuring the phosphorylation state of ERK1 / 2 , JNK and p38 MAP kinase as described in Material and Methods .