HPGDS and neoplasm: As GSTP1 is the major GSTs consistently expressed in both normal and tumour breast tissue (Forester, Carcinogenesis 1990; 11: 2163–2170), it can be hypothesised that low GSTP1 expression would reduce the global activity of GSTs, and consequently reduce glutathione (GSH) consumption in GST-catalysed reactions, thereby leading to higher levels of GSH, which would block apoptosis and promote proliferation of tumour cells.