Photograph of native chromatin preparation with fragments of, on average, one to five nucleosomes in length. For this experiment, nuclei were purified from primary fibroblasts and incubated with MNase for 6, 9, 12, and 15 minutes (lanes 1-4, respectively). The S1 fractions were obtained directly after MNase digestion, whereas the S2 fractions were recovered by overnight dialysis. Bands corresponding to chromatin fragments of one nucleosome (mono) to five nucleosomes (penta) in length are indicated (1.2% agarose gel). Fractions 1 and 2 of S1 were combined with fractions 3 and 4 of S2 for subsequent ChIP.