MicroRNA genes are highly associated with chromosomal features involved in the etiology of different cancers. The perturbations in the genomic structure or chromosomal architecture of a cell caused by these cancer-associated chromosomal features can affect the expression of the miR gene(s) located in close proximity to that chromosomal feature. Evaluation of miR gene expression can therefore be used to indicate the presence of a cancer-causing chromosomal lesion in a subject. As the change in miR gene expression level caused by a cancer-associated chromosomal feature may also contribute to cancerigenesis, a given cancer can be treated by restoring the level of miR gene expression to normal. microRNA expression profiling can be used to diagnose cancer and predict whether a particular cancer is associated with an adverse prognosis. The identification of specific mutations associated with genomic regions that harbor miR genes in CLL patients provides a means for diagnosing CLL and possibly other cancers.