Children with constitutional mismatch repair deficiency syndrome (CMMRDS) are prone to
different types of cancers. A 16-year-old girl who was misdiagnosed with neurofibromatosis
Type-I (NF-I) for 1 years had experienced glioblastoma and colonic adenocarcinoma. After
operation, chemotherapy and radiotherapy were started for adenocarcinoma. Genetic analysis
from the patient, effected brother, and mother showed heterozygote (c.479 + 36A> G) mutation
in the intron 4 region of NF-1 gene. Initially, it was thought that this genetic variant was
causative. Furthermore, next generation sequencing showed that the index patient and his
brother have homozygote (c.1444 C>T) mutations in the MSH6 gene which are associated with
CMMRDS both died because of colonic adenocarcinoma, and T cell non-Hodgkin lymphoma,
respectively. Patients with CMMRDS may resemble NF-I. The physicians must not be confused
with the previous diagnosis. Increased awareness of CMMRDS, and prompt evaluation for an
underlying genetic background is advised if there are unexpected cancer in patients with NF-I.