However, in a polyoma virus middle T antigen (PyMT)-driven allograft model of estrogen receptor (ER)-negative breast cancer, we found that while Annexin A1 expression was dispensable for cell survival and proliferation in vitro, it was unexpectedly required for the initiation of tumor growth in vivo and for the maintenance of a luminal mammary epithelial gene expression program. This evidence concerns the gene ANXA1 and neoplasm.