Deletions and mutations of PTEN/MMAC1 have been reported in a wide variety of human cancers, including tumours of the brain, the lung, the prostate and the breast (Teng et al, 1997; Bonneau and Longy, 2000; di Cristofano and Pandolfino, 2000), placing this gene among the most commonly mutated genes in human cancer. The gene discussed is PTEN; the disease is cancer.