  We study the spontaneous ''sliding'' of histone spools (nucleosomes) along
DNA as a result of thermally activated single base pair twist defects. To this
end we map the system onto a suitably extended Frenkel-Kontorova model.
Combining results from several recent experiments we are able to estimate the
nucleosome mobility without adjustable parameters. Our model shows also how the
local mobility is intimately linked to the underlying base pair sequence.
