In a melanoma lung metastasis mouse model, Ma et al. demonstrated that CHI3L1 upregulates the expression of programmed death-ligand 1 (PD-L1) on activated macrophages while concurrently suppressing the expression of Inducible T-cell Co-Stimulator (ICOS), ICOS Ligand, and CD28 on T cells and antigen-presenting cells [145,146]. The gene discussed is ICOS; the disease is melanoma.