The key to this particular model is in how the enzyme is made. Instead of relying on normal cellular processes, the team ensured thymidine kinase only appeared in cells capable of making another type of protein, known as surfactant protein B (SPB). They did this by replacing the genetic code promoting thymidine kinase production with the code specific for SPB. The team chose SPB as only two cell types in the body, alveolar type II and Clara cells, make the protein. These are also usual cell types associated with lung cancer.