In one such cell line, XIST was inserted into a copy of chromosome 21 in trisomy 21 iPSCs (Tri21 iPSCs), and a long noncoding RNA induced a series of chromatin modifications that stably silenced gene transcription across the whole chromosome in cis. Chromosome silencing occurred even in differentiated cells, and various pathologies observed in DS (including proliferative defects, impaired neural differentiation, and haematopoietic abnormalities) were successfully reversed by transcriptional inactivation of the supernumerary chromosome21,22. The gene discussed is XIST; the disease is Dravet syndrome.