The hippo pathway and its downstream mediator yes-associated protein 1 (YAP1) regulate mammalian organ size in part through modulating progenitor cell numbers. YAP1 has also been implicated as an oncogene in multiple human cancers. Currently, little is known about the expression of YAP1 either in normal human brain tissue or in central nervous system neoplasms. We used immunohistochemistry to evaluate nuclear YAP1 expression in the fetal and normal adult human brains and in 264 brain tumors. YAP1 was expressed infetal and adult brain regions known to harbor neural progenitor cells, but there was little YAP1 immunoreactivity in the adult cerebral cortex. YAP1 protein was also readily detected in the nuclei ofhuman brain tumors. In medulloblastoma, the expression varied between histologic subtypes and was most prominent in nodular/desmoplastic tumors. In gliomas, it was frequently expressed in infiltrating astrocytomas and oligodendrogliomas but rarely in pilocytic astrocytomas. Using a loss-of-function approach, we show thatYAP1 promoted growth of glioblastoma cell lines in vitro. High levels of YAP1 messenger RNA expression were associated with aggressive molecular subsets of glioblastoma and with a nonsignificanttrend toward reduced mean survival in human astrocytoma patients. These findings suggest that YAP1 may play an important rolein normal human brain development and that it could represent a new target in human brain tumors.