S100A9 and atrial fibrillation: ​​Importantly, while S100A9 CKO alone did not affect baseline AF susceptibility, it significantly reduced AF inducibility in the AF model group (Figures 4D, E).​​ Similarly, AF duration was markedly prolonged in the AF model group versus controls, but substantially shortened in AF model + S100A9 CKO mice (Figure 4F).