GSTs are believed to play a role in cancer prevention, as the electrophilic compounds that are conjugated to glutathione could otherwise react with and modify cellular components such as DNA, thereby initiating carcinogenesis. On the other hand, GSTs might detoxify chemotherapeutic drugs in cancer cells, thus contributing to drug resistance.
In summary, GSTs are a group of enzymes which are centrally involved in drug metabolism and detoxification. Because of their role in detoxification, they have been implicated in drug sensitivity and resistance. They may also influence mutagenesis and carcinogenesis and thus, increased GST activity may become a useful cancer marker.
Since many GST genes are polymorphic, there has been considerable interest in determining whether particular allelic variants are also associated with an increase of risk for certain diseases.

Glutathione S-transferase-Pi
The GST-Pi class (phase II detoxification enzymes) is the most abundant of the human glutathione S-transferases family, a major group of detoxification enzymes and widely distributed in the human body.
In normal human tissues, the enzymes protect cells against reactions with glutathione and reactive oxygen species by reducing organic hydroperoxides via gluthathione peroxidase activity.
The role of GST-Pi in tumor growth and progression is less well known. It is reported to be variably expressed in breast cancer and is associated with estrogen receptor levels expressed by the tumor. Down-regulation of GST-Pi activity in a study done on a T cell line appears to favor apoptosis and inhibition of GST-Pi function induces apoptosis in rat hepatoma cells.
The importance in cancer research is clear, as GST-Pi has been implicated in protection against apoptosis and also suggested to have a role in jun kinase inhibition.
The majority of human tumor cell lines express significant amounts of class pi GST, which is why it is being investigated as a potential marker for various types of cancer. GST-Pi is thought to be associated with increased resistance to anticancer drugs. GST-Pi expression is under investigation as a prognostic indicator for resistance to chemotherapy and a marker of treatment resistance.