The Carcinogenesis Pathway. Prolonged disruption of cellular function can cause DNA damage. Damaged cells initiate repair mechanisms such as base excision repair, nucleotide excision repair, mismatch repair, homologous recombination, and nonhomologous end joining. If repair fails, apoptosis eliminates the damaged cell, preventing further carcinogenesis. Defective DNA repair and apoptosis characterize the initiation stage. The initiated cell evades tumor suppressor genes and undergoes uncontrolled proliferation. With additional mutations, the cell becomes immortal and bypasses immune checkpoints. As mutations accumulate, the cell establishes a self-sustaining nutritional supply, promoting growth and leading to distant tissue invasion (metastasis). Contributed by SA Ibrahim, MBBCh, MSc, PhD