Three early studies have been published all of which demonstrated significant falls in Ki67 and evidence of decreased mTOR signalling.33–35 However, a recent larger, double-blind, placebo-controlled, randomised study showed no overall difference in expression of Ki67 or that of markers of the PI3K–mTOR and insulin signalling pathways.36 In prostate cancer, a window study recruited 24 patients and a diagnostic core biopsy was compared with a surgical specimen with a significant fall in Ki67 observed following metformin. Here, MKI67 is linked to prostate carcinoma.