The GST superfamily is made up of several subfamilies. Glutathione S-transferase P (GSTP1) belongs to the Pi subfamily and is involved in the conjugation of reduced glutathione to a variety of endogenous levels of and exogenous hydrophobic electrophiles. Glutathione S-transferase Mu 1 (GSTM1) is a cytoplasmic liver protein belonging to the Mu family and has the same basic functions as GSTP1-1. Glutathione S-transferase theta 1 (GSTT1), a cytoplasmic homodimer belonging to the Theta family, is expressed in erythrocytes. It is active in the reduced glutathione conjugation and also displays glutathione peroxidase activity with cumene hydroperoxide.