AIM: Toassess mitotic stability ofthe fragile X fullmutations andits relationshipwith DNA methylation. METHODS: The length change of the expanded CGG repeats was examined and correlated it with the methylation status in the DNA samplesisolated fromthe fibroblasts derived from a fragile Xfemale fetus anda fragileX maleadult, respectively. RESULTS: A dramatic instability of the expanded CGG repeats in the female fetal fibroblasts was observed. South- ern blot analysis revealed that the 6.9-kb major expanded band detected in passage 2 was completely replaced by a 7.7-kb band after passage 30. Fibroblast clones derived from the passage 3 displayed an unstable expansion of the CGG repeat during clonal proliferation, while methylation status of the CGG repeat region was maintained. In contrast, in fragile X male fibroblasts the expanded CGG repeats were stable during clonal proliferation. CONCLUSION: The mitotic instability of expanded CGG repeat is not always restricted in early development window as proposed previously and other elements rather than DNA methylation could affect the stability of the expanded CGG repeats in fragile X female fetal fibroblast cells. AIM: Toassess mitotic stability of the fragile X fullmutations and relationships with DNA methylation. Respectively. METHODS: The length change of the expanded CGG repeats was examined and correlated with the methylation status in the DNA samplesisolated from the fibroblasts derived from a fragile Xfemale fetus anda fragileX maleadult, respectively . RESULTS: A dramatic instability of the expanded CGG repeats in the female fetal fibroblasts was observed. South- ern blot analysis revealed that the 6.9-kb major expanded band detected in passage 2 was completely replaced by a 7.7-kb band after passage 30. [ Fibroblast clones derived from the passage 3 displayed an unstable expansion of the CGG repeat during clonal proliferation, while methylation status of the CGG repeat region was maintained. In contrast, in fragile X male fibroblasts the expanded CGG repeats were stable during clonal proliferation. CONCLUSION: The mitotic instability of expanded CGG repeat is not always restricted in early dev elopment window as proposed previously and other elements rather than DNA methylation could affect the stability of the expanded CGG repeats in fragile X female fetal fibroblast cells.