(A) Comparison between mouse and human Xist gene structure and tandem repeats. Core Repeat D and its extended truncated motifs are marked in dark and light green, respectively. (B) Outline of the putative role of human XIST repeats for XCI maintenance assessed in female K562 lymphoblast cell line and for XCI initiation in male HT1080 fibrosarcoma cell line with a Xist cDNA autosomal transgene; see main text for details.