AQP1 and chronic kidney disease: When the relationship between p16 staining and age was assessed in all cats without CKD, there was a significant correlation between p16 staining and age in the AQP1+ cell populations in both the renal cortex (p = 0.0006; r = 0.56) and corticomedullary junction (p = 0.002; r = 0.52; Figure 1), as well as in the AQP1− cell population in the renal cortex (p = 0.008; r = 0.45).