A study conducted using suicide gene transfer by a cytotoxic approach examined a way to eradicate malignant tumors.[115] Gene therapy is based on the cytotoxic genes that directly or indirectly kill tumor cells regardless of its gene expression. In this case the study uses the transfer of the Herpes simplex virus type I thymidine kinase (HSVtk) as the cytotoxic gene. Hela cells were used in these studies because they have very little ability to communicate through gap junctions.[116] The Hela cells involved were grown in a monolayer culture and then infected with the HSV virus.