SMAD3 and breast cancer: Supporting the sensitivity of the BioID method, we recovered several previously known interaction partners of TOP2B: TOP2A forms active heterodimers with TOP2B in HeLa cells [33]; TOP1 forms the DNA synthesome complex with TOP2B during DNA replication [34]; CTCF has been previously shown to interact with TOP2B in human breast cancer cell lines [35]; and ZNF451, a Smad3/4 transcriptional co-repressor [36] has been previously co-purified with TOP2B using tandem affinity purification mass-spectrometry [37].