IC followed by tumor promoters did cause tumors. A reversal of order, TP followed by IC produced no tumor. Something must happen first in order to start the process. This initiation lasts forever. The TP can be introduced immediately following exposure to the IC or at any time and the tumors will occur with the same frequency.
Cancer is a multistep process.
A normal cell must undergo the first step or initiation. This must be followed by promotion and a latency period before the development of a tumor. The initiators are chemicals that cause mutations. It is assumed that the mutations caused by the initiators are in cellular oncogenes. The promoters stimulate replication and proliferation of the cell.