肿瘤形成源于细胞转化。多种多样的化学致癌物如苯、吸烟、亚硝酸盐都能够引起(和)或促进肿瘤的发生。不管是周围环境中低水平长期的γ射线的作用，还是高剂量的放疗都会导致基因突变。感染性因素如人类乳头状瘤病毒能导致细胞转化。
伴随DNA改变的基因损伤会导致基因的点突变、染色体间基因移位和扩增性基因复制。这些改变能促使原癌基因改变为癌基因。原癌基因或许在胚胎期正常细胞的增生和调节起着非常重要的作用，然而成年人原癌基因处于“关闭”状态，但是通过转化可以使其启动。
The process of neoplasia begins with cell transformation. A variety of chemical carcinogens as diverse as benzene, cigarette smoke, and nitrites can initiate and/or promote this process. Radiation, either as low level long-term environmental gamma rays or as higher dose therapeutic radiation, can also produce genetic mutations. Infectious agents such as human papillomavirus can lead to cellular transformation as well.
Genetic damage with DNA alterations leads to point mutations of genes, translocations of genetic material between chromosomes, and gene reduplication with amplification. These alterations transform proto-oncogenes into oncogenes. The proto-oncogenes may play a role in growth promotion and regulation in normal cells, perhaps in embryogenesis, but are typically "turned off" in adults. They are "turned on" by transformation.