To determine whether up-regulated MCP-1 expression in tumor cells that metastasized to the lung was constitutive or in response to stimuli, we isolated tumor cells from well-isolated metastatic tumor nodules in the lung of WT or MCP-1−/− mice, depleting non-tumor cells and then examined the expression of MCP-1 mRNA in the resulting tumor cells. The gene discussed is CCL2; the disease is neoplasm.