Interestingly, both PMN-MDSCs and M-MDSCs upregulate AXL, MERTK and TYRO3, as well as their ligands GAS6 and PROS1 in a subcutaneous mouse model of melanoma.144 Consistent with this finding, the frequency of MERTK+ and TYRO3+ PMN-MDSCs and M-MDSCs was increased in patients with metastatic melanoma.144 Genetic ablation of Axl, Mertk or Tyro3 individually in mice led to reduced ARG1, TGFβ and ROS production in both types of MDSCs and iNOS and IDO is M-MDSCs.144 iNOS was also reduced in PMN-MDSCs in Axl−/− and Tyro3−/− mice and IDO in in PMN-MDSCs in Axl−/− and Mertk−/− mice. This evidence concerns the gene IDO1 and melanoma.