Neural and stem cell transplantation is emerging as a potential treatment for neurodegenerative diseases. Transplantation of specific committed neuroblasts (fetal neurons) to the adult brain provides such scientific exploration of these new potential therapies. Huntington's disease (HD) is a fatal, incurable autosomal dominant (CAG repeat expansion of huntingtin protein) neurodegenerative disorder with primary neuronal pathology within the caudate-putamen (striatum). In a clinical trial of human fetal striatal tissue transplantation, one patient died 18 months after transplantation from cardiovascular disease, and postmortem histological analysis demonstrated surviving transplanted cells with typical morphology of the developing striatum. Selective markers of both striatal projection and interneurons such as dopamine and c-AMP-related phosphoprotein, calretinin, acetylcholinesterase, choline acetyltransferase, tyrosine hydroxylase, calbindin, enkephalin, and substance P showed positive transplant regions clearly innervated by host tyrosine hydroxylase fibers. There was no histological evidence of immune rejection including microglia and macrophages. Notably, neuronal protein aggregates of mutated huntingtin, which is typical HD neuropathology, were not found within the transplanted fetal tissue. Thus, although there is a genetically predetermined process causing neuronal death within the HD striatum, implanted fetal neural cells lacking the mutant HD gene may be able to replace damaged host neurons and reconstitute damaged neuronal connections. This study demonstrates that grafts derived from human fetal striatal tissue can survive, develop, and are unaffected by the disease process, at least for 18 months, after transplantation into a patient with HD. R ecent findings in genetics, stem cell biology, and neural transplantation suggest that brain repair will be possible for the treatment of neurodegenerative diseases (1, 2). Before initiating large clinical trials that test the efficacy of novel donor cells, it is important to determine the clinical feasibility of such cell-based therapies. Transplantation of specific committed neuroblasts (fetal neurons) to the adult human brain provides such a scientific exploration of feasibility of cell-based therapies.The underlying genetic mutation of Huntington's disease (HD) is a polyglutamine repeat in the N-terminal region of the huntingtin gene (3). This mutation results in brain pathology dominated by massive neuronal loss of the medium spiny projection neurons of the caudate and putamen (4). Recent studies of HD postmortem brain tissue show that the N-terminal region of the mutant huntingtin protein aggregates in nuclear inclusions in both cortical and striatal neurons (5-7). These aggregates may represent evidence of ongoing cellular pathology (5-7). Implanted fetal neural cells lacking the mutant HD gene may be able to replace dead or dysfunctional host neurons and reconstitute disrupted neuronal connections (8, 9).Physiological and anatomical evidence in animal st...