Recent studies have demonstrated that resistance to trastuzumab treatment depends on the level of PTEN present (Crowder et al, 2004; Nagata et al, 2004; Pandolfi, 2004), with Nagata et al (2004) demonstrating that PTEN deficiency confers trastuzumab resistance in HER2/neu-overexpressing breast cancer cells. The gene discussed is ERBB2; the disease is breast carcinoma.