PDCD1 and melanoma: Preclinical studies reveal that variables such as cytokine milieu and expansion methods critically shape the repertoire, sometimes favoring in vitro-fit bystanders over bona fide tumor-reactive clones.123, 124, 125 Clinically, analyzing infused products’ repertoires has demonstrated that enrichment for tumor-derived clonotypes—rather than those originating from peripheral blood—predicts better responses in melanoma (with enrichment for tumor reactivity)116 and that increased clonality of expanded CD8+PD1+ T cells correlates with improved outcomes in advanced gastric cancer.119