Constitutional mismatch repair deficiency (CMMRD) syndrome, where a pathogenic germline variant is seen in both alleles of an MMR gene, also has been reported as a rare disorder and is known to be associated with diseases such as colorectal and small intestine cancer, acute leukemia, and brain tumors (medulloblastomas and high-grade gliomas) beginning in childhood [8]. The gene discussed is MRC1; the disease is acute leukemia.