We found that quercetin suppressed p300 HAT activity, abolished the p300-mediated NF-κB p50 acetylation, and blocked binding of p300 and the multiple transactivators such as CREB2, C-Jun, C/EBPβ and NF-κB to COX-2 gene promoter in human breast cancer cells. The gene discussed is NFKB1; the disease is breast cancer.