SPP1 and ischemia: To test the expression of both full‐length OPN (∼70 kDa) and active, N‐terminal OPN (N‐OPN, ∼50 kDa) in CKD, we constructed various CKD mouse models of unilateral ureteral obstruction (UUO), adriamycin nephropathy (ADR) and unilateral ischemia/reperfusion injury (UIRI).