Finally, despite FDA approval of pembrolizumab for treatment of microsatellite instability-high and mismatch repair-deficient cancers, interestingly we found that a number of DNA repair protein/phosphoprotein biomarkers, including phosphorylated BRCA1, CHK1, and CHK2, along with cleaved PARP; apoptosis/DNA damage markers cleaved caspase-3 and caspase-9 and phosphorylated gH2AX; and microsatellite instability markers MLH1, MSH6, and MSH2 did not associate with response to pembrolizumab in either TNBC or HR+/HER2− tumors in this study. Here, CHEK1 is linked to cancer.