Therefore, we firstly, for our knowledge, investigated the possibility of intratumoral ifosfamide turnover in human breast cancer in vitro using microsomal preparations from breast cancer tissue and highly sensitive HPLC/UV and LC/MS detection to determine metabolites of ifosfamide bioactivation (4-hydroxylation) and N-dechloroethylation as well as Western Blotting to detect the expression of CYP3A4, CYP2C9 and CYP2B6 in the tumour samples. This evidence concerns the gene CYP2C9 and breast cancer.