For many years, it has been thought that the chromosome region 10q22-24 includes one or more genes that appear to play a role in several human malignancies. PTEN is a new tumor suppressor gene encoding a dual-specificity phosphatase that was cloned simultaneously by three groups (Li & Sun 1997, Li et al. 1997, Steck et al. 1997), two of which used a positional cloning approach to identify genes in chromosome 10 (Li et al. 1997, Steck et al. 1997). While several protein kinases have been implicated as oncogenes, and phosphatases have long been known frequently to antagonize their function, there has been no direct demonstration of the role of phosphatases in tumor development (Myers & Tonks 1997). PTEN characterization as a bona fide tumor suppressor gene has confirmed that a deficient phosphatase activity can lead to cancer, as detailed by studies that are described below.