An autosomal recessive constitutional mismatch repair deficiency syndrome caused by pathogenic variants in the MLH1 mismatch repair gene. It is characterized by a high risk of childhood cancers, including hematological malignancies and brain tumors, as well as colorectal cancers with polyposis. The disease is mismatch repair cancer syndrome 1 (MONDO_0010159). Also known as: BTP1 syndrome, MLH1-related constitutional mismatch repair deficiency syndrome, MMRCS1, Turcot Syndrome, brain tumor-polyposis syndrome 1.