We further show that the oncogenic activities of BORG are mediated in part through its binding to the E3 SUMO ligase, TRIM28 (KAP1), which functions as a (i) transcriptional co-repressor and scaffolding protein for histone and DNA modifying enzymes; and (ii) stimulator of breast cancer proliferation by suppressing the transcription of Cdkn1a (p21) and Gadd45a15–18. Here, CDKN1A is linked to breast cancer.