Using transgenic mice expressing the polyoma middle T antigen under the control of MMTV (mouse mammary tumor virus) long terminal repeat promoter (MMTV-PyMT), the authors found that heterozygous mouse for KiSS-1 or KiSS-1R showed delayed hyperplasia, resulting in a late breast cancer initiation, progression, and lung metastasis. Here, KISS1 is linked to breast cancer.