GAPDH and urinary bladder carcinoma: Differential expression of two often-used housekeeping genes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and β-actin (ACTB), in hypoxia has repeatedly been reported in vitro and in vivo: GAPDH belongs to the least stably expressed genes in human retinal endothelial cells [7] and bladder cancer cells [8] in hypoxia; its protein levels also increase to 3–4-fold in rat alveolar epithelial cells after 18 h of hypoxia [9].