DKK1 and central nervous system cancer: For instance, in the aforementioned morphological changes induced by the Wnt signaling pathway, it has been reported that DKK-1 expression is enhanced in glioma cells [28] and bone [29] when the partial pressure of oxygen is decreased, and it is possible that differences in oxygen conditions caused by the difference of the materials of chips or perfusion rate of the medium, therefore, affected DKK-1 secretion.