To assess the role of ERCC6L in breast tumorigenesis in vivo, a spontaneous breast cancer mouse model was constructed by crossing mice of three genotypes with MMTV-PyMT mice, named ERCC6L+/+ PyMT, ERCC6L+/− PyMT and ERCC6L−/− PyMT (Fig. 5A). The gene discussed is ERCC6L; the disease is breast cancer.