In ovarian cancer, loss-of-heterozygosity (LOH) at the PTEN locus (10q23.3) occurs in 31–45% of tumours, whereas mutations of the second PTEN allele are relatively rare (Maxwell et al, 1998; Obata et al, 1998; Kurose et al, 2001). The gene discussed is PTEN; the disease is ovarian cancer.