When the expression of checkpoint antigens on cancer cells was considered, a statistically higher proportion of PD-L1 and PD-L2 expression was demonstrated in NSCLC patients than in SCLC patients (respectively, 21.6 vs. 6.4%, p = 0.046, and 58.7 vs. 26.7%, p < 0.0001), as well as a statistically lower proportion of CTLA-4 expression in NSCLC patients than in SCLC patients (respectively, 3.2 vs. 21.4%, p = 0.0122) (Figure 6, Table 4). Here, CTLA4 is linked to cancer.