It is now well accepted that tumors are able to evade detection and eradication by the immune system, even though many tumor types, particularly melanoma, are capable of eliciting a strong immune response (Swann, 2007). Substantial mechanistic work in recent years has revealed the key role of myeloid-derived suppressor cells (MDSCs) in masking cancer cells from the immune system, promoting both tumor progression and resistance to cancer immunotherapy. The immune-suppressive effect of MDSCs is dependent on the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS). High levels of these reactive molecules and their by-products, such as nitrotyrosine, have been correlated with poor clinical outcomes in melanoma. Currently available melanoma therapies do not target MDSCs.