Altered nucleosome spacing associated with Down syndrome.

Micrococcal nuclease digestions of human leukocytes were analyzed using gel electrophoresis. The relative size (or average number of nucleotides) of nucleosomes was determined. The mean nucleosome size of the normal 46 chromosome karyotype is 185 +/- 2 bp. There was no significant variation in nucleosome size within the normal population due to age, sex, or race. The mean nucleosome size of the Down syndrome samples is 173 +/- 3 bp. These two groups have separate normal distribution curves which do not overlap and are significantly different at P < 0.1%.