Retinoblastoma is a pediatric retinal tumor caused by mutational inactivation of the tumor suppressor pRb. Additional genetic changes, as yet unidentified, are believed to be required for tumor initiation. Mutations in the Writ signaling pathway have been implicated in the pathogenesis of many cancers. Multiple Wnt pathway genes are expressed in the retina and the pRb and Win pathways interact biochemically, raising the possibility that alterations in the Wnt pathway contribute to retinoblastoma. Our studies showed that Writ signaling activation significantly decreased the viability of retinoblastoma cell lines by inducing cell cycle arrest, which was associated with upregulated p53. Furthermore, immunolocalization of the Wnt signaling mediator P-catenin in human and mouse retinoblastoma tissue indicated that canonical Wnt signaling is suppressed in tumors in vivo. These studies are consistent with the Wnt pathway acting as a tumor suppressor in retinoblastoma and suggest that loss of Wnt signaling is tumorigenic in the retina. (c) 2006 Elsevier Inc. All rights reserved.