Leukocytosis in solid tumors can be caused directly by tumor cells via secretion of stimulatory factors such as granulocyte-colony-stimulating factor (G-CSF), granulocyte-macrophage-colony-stimulating factor, and upregulation of C-X-C chemokine receptor 2 (CXCR2) ligands (17). The gene discussed is CSF3; the disease is neoplasm.