As a surrogate of IDO-1 level in the TME, peripheral blood levels of tryptophan (Trp) and kynurenine (Kyn) have also been measured, and a high serum Kyn/Trp ratio appears to be associated with shorter PFS in cHL and correlated with high levels of IDO-1 IHC expression in the TME [69]. The gene discussed is IDO1; the disease is classic Hodgkin lymphoma.