INS and obesity due to melanocortin 4 receptor deficiency: Endometrioid endometrial or Type 1 cancers represent 80% of all endometrial tumours and are thought to be driven by obesity and insulin resistance,32 hence the potential for metformin to act on insulin-dependent cancer signalling pathways.7 Metformin has shown significant promise as an anti-cancer treatment in various pre-clinical studies25–27 and this has been recapitulated in single arm clinical trials.8,10,13,14 The response to metformin, however, varied between patient groups.