Tumor development has long been known to resemble abnormal embryogenesis. The 
embryonic stem cell (ESC) self-renewal gene NANOG is purportedly expressed by 
some epithelial cancer cells but a causal role in tumor development has remained 
unclear. Here, we provide compelling evidence that cultured cancer cells, as 
well as xenograft- and human primary prostate cancer cells express a functional 
variant of NANOG. NANOG mRNA in cancer cells is derived predominantly from a 
retrogene locus termed NANOGP8. NANOG protein is detectable in the nucleus of 
cancer cells and is expressed higher in patient prostate tumors than matched 
benign tissues. NANOGP8 mRNA and/or NANOG protein levels are enriched in 
putative cancer stem/progenitor cell populations. Importantly, extensive 
loss-of-function analysis reveals that RNA interference-mediated NANOG knockdown 
inhibits tumor development, establishing a functional significance for NANOG 
expression in cancer cells. Nanog short hairpin RNA transduced cancer cells 
exhibit decreased long-term clonal and clonogenic growth, reduced proliferation 
and, in some cases, altered differentiation. Thus, our results demonstrate that 
NANOG, a cell-fate regulatory molecule known to be important for ESC 
self-renewal, also plays a novel role in tumor development.