Unexpectedly, established AKI subjects displayed substantially elevated kallikrein excretion (Figure 2, Table 1), about ~10 times higher than that of controls (activity: 6132.9 ± 2302 vs. 623.0 ± 88.2 mU/L, p < 0.001; urine kallikrein activity/creatinine ratio: 6.74 ± 1.92 vs. 0.63 ± 0.08 U/gm, p < 0.001). This evidence concerns the gene KLK4 and acute kidney injury.