Transplanted bone marrow derived cells (BMDCs) have been reported to fuse with cells of diverse tissues [1][2][3][4][5][6][7][8][9][10][11][12][13] , but the extremely low frequency of fusion has led to the view that such events are biologically insignificant. Nonetheless, in mice with a lethal recessive liver disease (tyrosinaemia), transplantation of wild type BMDCs restored liver function by cell fusion and rescued the mice from death 3, 9 , indicating that cell fusion can have beneficial effects. Here we report that chronic inflammation resulting from severe dermatitis or autoimmune encephalitis leads to robust fusion of BMDCs with Purkinje neurons and formation of hundreds of binucleate heterokaryons, a 10-100 fold higher frequency than previously reported 8,10,11,14 . Single haematopoietic stem cell transplants showed that the fusogenic cell is in the haematopoietic lineage and parabiosis experiments revealed that fusion can occur without irradiation. Species-mismatched bone marrow transplants resulted in activation of dormant rat Purkinje neuron-specific genes in BMDC nuclei post-fusion with mouse Purkinje neurons consistent with nuclear reprogramming. The precise neurological role of these heterokaryons awaits elucidation, but their frequency in brain after inflammation is clearly much higher than previously appreciated.Although fusion of like cells has long been known to accompany the normal development of a number of tissues such as skeletal muscle, bone and the placenta 15-17 , recent evidence from numerous laboratories indicates that bone marrow derived cells (BMDC) can fuse with