Hypoxia, defined as in decreased oxygen tension than physiological range, triggers numerous cellular responses via hypoxia-inducible factor (HIF). As for kidney, hypoxia in tubulointerstitial area aggravates CKD progression, therefore tubular hypoxia and HIF activity as a response to hypoxia are attracting more attention. Recently, the existence of oxygen gradient in the mouse renal cortex was reported (Hirakawa Y, Kidney Int 2018). Therefore there is a need for the research on HIF activity with oxygen gradient.