CXCL13 and breast carcinoma: Notably, a recent study employing single-cell RNA sequencing analysis revealed that expression of PD-1 and CXCL13 on T cells and MHC-I (major histocompatibility complex I) on cancer cells could classify luminal breast cancer patients with respect to T cell exhaustion phenotype [47], denoting the complexity of studying T cell exhaustion in TME.