Although loss of heterozygosity at the PTEN locus is a very common event in muscle-invasive transitional cell carcinomas of the bladder [33], previous attempts to generate mouse models based upon Pten deficiency have shown rather mild phenotypes, varying from an incidence of 10% of transitional cell carcinomas [34] to no tumorigenesis at all [35], [36]. Here, PTEN is linked to transitional cell carcinoma.