Currently, carcinogenesis is considered to be the result of mal-expression of 
tumour suppressor genes and oncogenes, leading either way to uncontrollable and 
disorganized cell mitosis. Recently a novel class of genes has drawn the 
interest of the scientific community. These are microRNAs (miRNAs), a class of 
noncoding RNAs, 20-23 nucleotides in length, that can up or downregulate gene 
expression of downstream gene targets (including transcription factors, 
oncogenes, and tumour suppressor genes) at the post-transcriptional level. Some 
members of this new class of genes seem to have the potential to act 
simultaneously either as oncogenes or as tumour suppressor genes depending on 
the molecular microenvironment of the cell. We elaborate on this hypothesis by 
giving examples of miRNAs (e.g. mir-9, miR-17-92) which seem to function by the 
abovementioned mechanism. This could mean that the deterministic notion of 
carcinogenesis as a result of merely tumour suppressor genes and oncogenes 
deregulation could be revised to contain the fact that certain members of this 
novel class of genes have the potential to play both roles simultaneously.