IDO1 and melanoma: Accordingly, in breast cancer patients, the number of IDO1-expressing MDSCs is increased and their IDO1 expression is positively associated with Foxp3+ Treg cell infiltration in tumours and lymph node metastases.53 Moreover, IDO1 expression in melanoma cells was shown to promote immunosuppression by expanding, recruiting and activating MDSCs in a Treg-dependent manner.54