Gap junction (GJ) channels directly facilitate the intercellular exchange of ions and small molecules including the toxic metabolites of GCV, which pass directly from HSV-TK expressing cells to surrounding cells that do not express it, so the bystander effect in suicide gene therapy is usually proportional to the degree of GJIC in cancer (Mesnil and Yamasaki, 2000; Matono et al., 2003; Xiao et al., 2017). The gene discussed is TKT; the disease is cancer.