PDCD1 and breast cancer: In breast cancer, CTLA-4 is highly co-expressed with PD-1, LAG-3, T-cell immunoglobulin and mucin-domain-containing-3 (TIM-3; encoded by HAVCR2), and T Cell Immunoreceptor With Ig And ITIM Domains (TIGIT; encoded by TIGIT) on tumor-infiltrating lymphocytes, suggesting that CTLA-4 blockade may synergize with other checkpoint inhibitors [139].