Studies to date show the safety of using autologous  human mesenchymal stem cells for bone regeneration in segmental long-bone defects. None of the reports mention adverse effects, such as inflammation or excessive tissue growth. In view of the report that extensively cultured mesenchymal stem cells (4-5 months) can develop genomic instability, an indicator of malignant transformation, the relatively short in vitro expansion period (6-8 weeks) may account for the lack of reported malignancy in the clinical studies performed so far.