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It is well known that the circadian clock system coordinates physiological functions throughout the day (Dibner et al., 2010; Mo-hawk et al., 2012; Fu and Lee, 2003; Ham-mad et al., 2013). Circadian rhythms are generated by molecular feedback loops; tran-scriptional activators induce the expression of genes that repress their own transcription (Lowrey and Takahashi, 2011). Disruption of circadian rhythms has been discussed as a possible risk factor of cancer. For example, epidemiological studies have suggested that working at night increases the risk of breast cancer (Kamdar et al., 2013). However, re-cently a comprehensive study including 766 breast cancer patients has been published that broadens our understanding of the role of the circadian clock system in breast cancer (Cadenas et al., 2014). The authors studied all known clock genes in relation to progno-sis. Interestingly, loss of expression of circa-dian clock genes in tumour tissue was asso-ciated with a clearly higher risk to develop metastasis (Cadenas et al., 2014). Recently, numerous studies have been performed to identify prognostic genes in breast cancer (e.g. Sicking et al., 2014a, b; Siggelkow e