This systematic review was conducted to (1) estimate the prevalence of MMR deficiency (dMMR) and confirmed LS in patients diagnosed with CRC before the age of 50, and (2) examine immunohistochemistry (IHC) mismatch repair testing patterns and associated molecular findings (BRAF mutations, MLH1 promoter hypermethylation, somatic MMR gene alterations). This evidence concerns the gene MLH1 and mismatch repair cancer syndrome 1.