S100A9 and infection: Conversely, gene clusters showing the highest expression in C3HeB/FeJ mice were only substantially increased by 21 days after infection and were dominated by neutrophil and inflammatory myeloid-associated genes such as Ly6g, Cxcr2, S100a8, and S100a9 (Fig. 1 d, cluster 6; Fig. 1 f, cluster 5).