MRC1 and mismatch repair cancer syndrome 1: Some possible explanations for improved survival with MMR deficiency in TNBC include: (1) MMR deficiency was most common in these tumors, (2) immune check point inhibitors are commonly used in this cancer subtype, and (3) MMR-deficient tumors have been shown to respond to checkpoint point inhibition across many cancer subtypes, although breast was not studied [31].