Venet et al. derived a general index of proliferation, ‘metaPCNA’, by identifying the top 1% of genes most positively correlated with the proliferation marker PCNA (proliferating cell nuclear antigen) across 36 healthy tissue types and demonstrated that it significantly outperformed a majority of prognostic signatures developed for breast cancer (Supplementary Table 1) [27, 28]. Here, PCNA is linked to breast cancer.