TP53 and cancer: For example, to infer the role of the gene TP53 in cancer, one can compare standardized mice, some with an inactivated form of TP53 and some with functioning TP53. If mice with inactivated TP53 show abnormal cell development that results in tumors, whereas mice with functioning TP53 show normal cell development, then—assuming all other factors are equal5—we can infer that the TP53 gene plays a causal role in cell development (Mitchell 2009).