Drawing comparisons with other HER2-driven malignancies, opposite results have been reported: in gastric cancer—despite relying on PD-L1 as a biomarker rather than MMR status—significant results have been reported in first-line treatment, with pembrolizumab and trastuzumab becoming the standard of care following the results of the KEYNOTE-811 trial [22]; conversely, in breast cancer, the addition of immunotherapy to HER2-directed treatment has not demonstrated clinical benefit in either early-stage disease [23]. This evidence concerns the gene ERBB2 and breast carcinoma.