Using a spontaneous metastasis model (fat pad injection of cancer cells), they showed that paclitaxel exacerbated the ability of breast cancer cells to metastasize in the wild type (WT) mice, but not much in the knockout (KO) mice deficient in Atf3. Since the same breast cancer cells were injected into the mice and the only difference was the host, it means that paclitaxel exerted its pro-metastatic effect by affecting the host cells. The gene discussed is ATF3; the disease is cancer.