IDO1 and melanoma: Despite pre-clinical evidence (Spranger et al., 2014; Triplett et al., 2018) and promising results in several Phase I/II clinical trials (Gibney et al., 2015; Gangadhar et al., 2017; Hamid et al., 2017; Daud et al., 2018; Mitchell et al., 2018; Zakharia et al., 2018), an IDO-1 inhibitor failed to show clinical benefit in a Phase III clinical trial in melanoma (Long et al., 2019).