This invention pertains to the discovery that an amplification of the CYP24 gene or an increase in CYP24 activity is a marker for the presence of, progression of, or predisposition to, a cancer (e.g., breast cancer). Using this information, this invention provides methods of detecting a predisposition to cancer in an animal. The methods involve (i) providing a biological sample from an animal (e.g. a human patient); (ii) detecting the level of CYP24 within the biological sample; and (iii) comparing the level of CYP24 with a level of CYP24 in a control sample taken from a normal, cancer-free tissue where an increased level of CYP24 in the biological sample compared to the level of CYP24 in the control sample indicates the presence of said cancer in said animal.