How the functioning of X chromosomes differs in women and men may help to explain biological differences between the sexes, according to a new study by researchers from Duke and Pennsylvania State Universities.

The researchers, writing today in the journal Nature, said the results implied that women make higher doses of certain proteins than men, which could result in differences in both normal life and disease.

Women turn off one copy of their X chromosome in each cell, so that, like men, they operate with just one copy functioning. But scientists had long known that the inactivation was not complete.

The study found that 15 percent of the genes on the inactivated copy continued to function, sending out chemical orders for the cell to manufacture specific proteins.

Researchers also found about another 10 percent of the genes in which the activity level varied widely by woman, from zero in some to varying levels in others. That contrasted with the more consistent activity levels in X chromosomes from men, or in other chromosomes in either sex.