Australian scientists have shown how a ‘transcription factor’ causes breast cancer to develop an aggressive subtype that lacks sensitivity to oestrogen and does not respond to anti-oestrogen therapies such as Tamoxifen and aromatase inhibitors. Transcription factors are molecules that switch genes on or off. In this case, the transcription factor known as ‘ELF5' inhibits sensitivity to oestrogen very early in the life of a breast cancer cell.