However, in 2001, the Jeffrey Pollard group showed that mice with a recessive null mutation in the colony-stimulating factor 1 gene (Csf1op), the major macrophage growth factor, and genetically modified to develop mammary cancer had a delay in the development of metastatic carcinomas, therefore showing the involvement of macrophages in malignant progression of breast cancer [35]. The gene discussed is CSF1; the disease is breast cancer.