The Sleeping Beauty transposon has emerged as a new, powerful tool in cancer research (reviewed in PMID:26051241). The transposon mutagenesis screen is a system-biological approach to decipher the regulatory networks that normally suppress cancer development. This strategy is able to identify both oncogenes and tumor-suppressor genes. We utilize the hyperactive SB100X system in a rat model to decipher gene regulatory networks cooperating in hormone-dependent breast cancer development, progression (collab. M. Bader (MDC), D. Largaespada (U. Minnesota), J. Shull (U. Wisconsin). Although, numerous studies implicate an association between estrogens and the development of breast cancer, the molecular mechanisms through which estrogens induce or promote breast cancer development are not clear. Unlike the situation in mouse, the development of mammary cancer in rat is similar to human as it is also estrogen-dependent.