Fanconi anemia (FA) is a monogenic inherited disorder mainly characterized by congenital abnormalities, bone marrow failure (BMF) and cancer predisposition. Although mutations in any of the 23 FA/BRCA genes can account for the disease, mutations in FANCA account for around 60-70% of FA patients worldwide. Currently, allogeneic hematopoietic stem cell (HSC) transplantation constitutes the only therapeutic approach capable of correcting the hematopoietic defects of these patients in the long-term. Nevertheless, side effects such as graft versus host disease, and also a significant increase in the development of squamous cell carcinomas in transplanted patients justify the necessity of developing safer therapies. Preclinical studies conducted in our laboratory have shown that in contrast to any other disease currently treated by gene therapy, gene corrected HSCs from FA-A patients develop a proliferation advantage after infusion in transplanted immunodeficient mice. Prior to the development of a gene therapy trial in FA-A patients, the conditions required for the efficient mobilization of CD34⁺ cells from these patients were optimized in the FANCOSTEM I trial. Thereafter, a phase I/II lentiviral-mediated clinical trial was proposed. Due to the proliferative advantage of corrected HSCs demonstrated in our preclinical studies, in the FANCOLEN I clinical trial gene corrected mPB CD34⁺ cells were infused in the patients in the absence of any pre-conditioning regimen. The results obtained up to a 5 years of follow-up in eight evaluable FA-A patients indicate that in most patients a slow, thought progressive engraftment of gene-corrected cells took place, which currently reached up to 70% corrected cells in one of the patients. Increases in the proportion of corrected cells were associated both with a progressive increase in the resistance of bone marrow progenitor cells to the genotoxic agent mitomycin C, and also with a reduction in the chromosomal instability in PB T cells exposed to diepoxibutane. Additionally, insertion site studies confirmed the safety of the gene therapy trial since no insertional oncogenesis events were observed. Stabilized, and even improved PB cell counts have been observed in patients showing higher levels of gene correction. Nevertheless, patients treated at advanced stages of the disease or infused with very low numbers of corrected CD34⁺ cells showed progressive evolution of BMF. Our results demonstrate that gene therapy of non-conditioned FA patients has the potential to prevent the progression of BMF. Based on these results, a global phase II clinical trial is currently ongoing under the sponsorship of Rocket Pharmaceuticals Inc. This trial is focused on the prevention of BMF by means of the infusion of higher numbers of corrected CD34⁺ cells in early stages of the disease.