Hereditary forms of colorectal cancers are rare, they provide about 5% of the whole colorectal cancers. The individuals carrying a major genetic factor predisposing to colorectal cancer are at major risk in the absence of adequate care. Lynch syndrome is the most frequent form of colorectal cancer, it leads to increased susceptibility to develop cancers. At the forefront we have colorectal cancer against Endometrial cancer which is lesser extent, as well as various cancers (ovarian, small intestine, stomach, bladder and urinary tract hepatological) and predisposes to a lifetime risk of 50 to 80% of developing colorectal cancer. Lynch syndrome of autosomal dominant transmission, is due to coexistence of constitutional and somatic mutations, causing a loss of protein expression involved in the DNA mismatch repair (MMR), genes whose alteration is associated with the existence of an HNPCC, and belongs to the family of, MSH2, MLH1, and MSH6, are involved in decreasing frequency in respectively 35%, 25% and 2%. As report case we will use Moroccan family carrying the germinal MLH1 mutation, which presents several cases of colon cancer. Indeed the molecular-genetic study for our patient colorectal cancer carrying, determined the presence of MLH1 heterozygote mutation.Following a request from his brother aged of 36 old , a molecular study during a ontogenetic consultation after his informed consent , we conclude that he carries the same mutation identified in his brother : deleterious heterozygote MLH1 mutation ,this positive presymptomatic test included developing monitoring process as recommended by the Lynch syndrome.