Cancer genes can be grouped into two classes: class I genes are mutated or deleted; whereas class II genes are not altered at the DNA level, rather they affect the phenotype by expression changes.
Conventionally, in the study of diseases like cancer, only mutated (class I) genes have been considered as candidate cancer genes, cancer genes of class II have been neglected by cancer geneticists. Until now the focus has been on class I genes and the screening methods have been largely limited to searches for alterations in DNA.
But cancer phenotypes result from altered gene expression (class II), and there is no simple 1:1 relationship between mutated genes and cancer phenotypes. Therefore, it’s not enough to know whether a gene is mutated or normal — it’s also important to know whether that gene is expressed and how it is expressed.
Both over-expressed and under-expressed genes can be identified in the one test, making it possible to recognise genes whose expression changes during development, carcinogenesis, or any other process under investigation.