The transformation of a normal cell to a cancer cell is a complex process that may take years. Drug-induced carcinogenesis usually begins with DNA damage. Reactive drugs or their metabolites can bind DNA, thereby causing frameshift or point mutations. When DNA repair mechanisms are unable to fix the mistake, changes in the DNA persist and undergo replication. When these mutations affect genes that control cell division, uncontrolled growth may be the result.
Two types of genes are of great importance for the transformation towards a tumor cell: proto-oncogenes and tumor suppressor genes. Mutation of a proto-oncogene can lead to an overexpression of proteins involved in cell growth and division. Tumor suppressor genes encode for proteins that inhibit cell division. Mutations in these kind of genes might thus inhibit their expression and decrease the production of cell-cycle inhibitory factors.
See also the chapter of oncology.