sentence: In some experiments , NIH3T3 cells constitutively expressing E7 genes were generated by infection with pBabepuro vector expressing HPV16 HA-tagged E7 proteins , wild-type or mutant.The 2BN11 cell line was created by infecting NIH3T3 cells with recombinant retrovirus expressing the HPV16 E7 gene under the control of a tetracycline repressed promoter .