TK1 and viral infectious disease: Furthermore, a few viral infections in humans, mainly caused by the family Herpesviridae (e.g., herpes simplex virus, Epstein‐Barr virus, varicella‐zoster virus and cytomegalovirus) and hepatotropic viruses, are associated with the increased activity of serum TK1 (Gronowitz et al., 1984; Tanaka et al., 1993).