IDO1 and breast cancer: In melanoma, combined PD-1 and CTLA-4 inhibition, while associated with significant toxicity, has been shown to be effective.49 Such combinations are yet to be adequately investigated in breast cancer (NCT02892734), as are other checkpoint proteins such as LAG-3.50 Another potential way to target anti-PD-1 resistant tumours may be via indoleamine 2,3-dioxygenase 1 (IDO1) inhibition,51 or by targeting the adensoine pathway52 (NCT02655822).