CD4 and neoplasm: Treatment of tumours consisting of HCmel12 CRISPR-control (ctrl) and HCmel12 Trp1-KO mixtures demonstrated that TRP-1 CD4+ T cells also exerted substantial bystander killing activity, but could not prevent the outgrowth of HCmel12 Trp1-KO cells in all mice (Extended Data Fig. 2q,r).