To elucidate whether S100A9−/− plays a key role in AD progression, we crossed F1 male S100A9+/−/Tg2576 (HT/Tg) mice with female S100A9+/−/WT (HT/WT) mice to generate F2 littermates with the following genotypes: S100A9+/+/WT (WT/WT), HT/WT, S100A9−/−/WT (KO/WT), S100A9+/+/Tg2576 (WT/Tg), HT/Tg and S100A9−/−/Tg2576 (KO/Tg) mice. The gene discussed is S100A9; the disease is Alzheimer disease.