The constitutional MisMatch Repair deficiency (CMMRD) syndrome is one of the inherited cancer predisposition syndromes. More than two-third patients belonging to a CMMRD family are diagnosed mainly in the first decade with brain cancers and/or hematological malignancies. This syndrome is due to bi-allelic germline mutations in genes of the MMR pathway (MLH1, MSH2, MSH6 or PMS2). Our family report begins with the index case presenting initially with a medulloblastoma, which was even the two relapses in complete remission, when she was diagnosed for an AML. She died after bone marrow transplantation from toxicity. The family history was progressively established when her uncle was diagnosed for a colonic cancer and a cousin for a brain tumor. Surprisingly, her father had an atypical sarcoma but her brother also presented a lymphoma followed by a gliomatosis cerebri. A new MLH1 bi-allelic mutation was identified in this family. More than the diagnostic difficulties, this family report illustrates the complexity of the microsatellite instability detection in CMMRD patients, which has to be discussed further to a more accurate diagnosis in the pediatric setting, and address the question of the proper diagnostic tool to use in such genetic background with hypermutated tumors.