Mdrl gene contributes to multidrug resistance.Because of the level of Mdrl gene expression is correlated with the response of chemotherapy,it is a valuable method to measure the expression level of individual patients. The cDNA cloning of the human Mdrl gene has made it possible to measure levels of Mdrl RNA with hybridization in human cancer cells. A special part of Mdrl cDNA was obtained by use of RT-pCR and inserting it into pUC18 vector.The DNA sequencing result is identical with the sequence of Mdrl cDNA reported. This probe can be used to detect the level of Mdrl gene expression in clinical specimen. The measurements can be useful in the design of chemotherapeutic protocols for certain tumors.